How do I get public key hash for SSL pinning?

Question

How would I get the hash of a public certificate's info to be able to perform SSL Pinning in my application?

I am using TrustKit in my iOS application and it is asking for me to set the hash that I am expecting. Where can I get this from?

Au

Also see OWASP's Certificate and Public Key Pinning. The web's rendition of pinning is a joke... – Unfix

Answer 1

If it is a public website, you can use SSL Labs server test which computes and displays the pin.

The Public Key Pinning page over at the Mozilla Developer Network also has commands for obtaining the pin from a key file, a certificate signing request, a certificate or a website (this is the one in @mylogon's answer).

Snarl

How can I find backup key? – Sullyprudhomme

you can also use dummy hash, try to connect to the server, and in log you will get right hash from the server – Verdieverdigris

Answer 2

Since this is a programming web-site, i wanted to explain how to calculate the public key hash for Public Key Pinning. This is important because the algorithm is completely undocumented. (It's not even documented by the canonical RFC 7469 Public Key Pinning Extension for HTTP ! The RFC simply says "Use OpenSSL". That is a non-answer.)

tl;dr: Base64(SHA256(SubjectPublicKeyInfo))

A Certificate is not (just) a public key

The first important thing to realize is that a public key is just a number (e.g. 2,048 bit number for RSA). A certificate is a collection of a lot of additional information (subject names, hashes, digital signatures).

This means that:

• a public key is a number
• a certificate associates a web-site with that number (e.g. stackoverflow.com)

A public-key pin contains a hash of the public key. It is not the hash of the certificate. This lets you renew certificates while keeping your same public key.

The Certificate structure

We need to extract the public key from a certificate, so we need to know its structure. The structure of a certificate is documented (horribly) in RFC 5280 - Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile

In programming terms, a Certificate is:

struct Certificate {
   TBSCertificate      tbsCertificate,
   AlgorithmIdentifier signatureAlgorithm,
   BITSTRING           signatureValue
}

And it's the SubjectPublicKeyInfo inside the tbsCertificate that contains what we're after:

struct TBSCertificate {
    EXPLICIT                version,
    CertificateSerialNumber serialNumber,
    AlgorithmIdentifier     signature,
    Name                    issuer,
    Validity                validity,
    Name                    subject,
    SubjectPublicKeyInfo    subjectPublicKeyInfo,
    //...optional extra stuff...
    }

We need to take the SHA256 of the raw subjectPublicKeyInfo.

digest = SHA256(certificate.tbsCertificate.subjectPublicKeyInfo);    

That value, base64 encoded, becomes the value we put in our web-server response headers:

String pin = String.Format("max-age=%d; pin-sha256=\"%s\"", 
    31536000, //1 year
    System.Convert.ToBase64(digest));
response.Headers.Add("Public-Key-Pins",  pin);

For example:

Public-Key-Pins: max-age=31536000; pin-sha256="hUIG87ch71EZQYhZBEkq2VKBLjhussUw7nR8wyuY7rY=";

But what do i actually hash?

As an example of calculating the PKP SHA256 hash, i am going to use the current certificate of Facebook.com. You can browse to Facebook, view the certificate, and save it in a base64 encoded DER PEM:

Facebook.cer (PEM DER ASN.1 encoded)

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

All the above is base64 encoded. Let's decode it into hex:

Facebook.cer (DER ASN.1 encoded)

30 82 07 e4 30 82 06 cc a0 03  02 01 02 02 10 0c 00 99 b7 d7  89 c9 f6 66 26 31 7e bc ea 7c  1c 30 0d 06 09 2a 86 48 86 f7 
0d 01 01 0b 05 00 30 70 31 0b  30 09 06 03 55 04 06 13 02 55  53 31 15 30 13 06 03 55 04 0a  13 0c 44 69 67 69 43 65 72 74 
20 49 6e 63 31 19 30 17 06 03  55 04 0b 13 10 77 77 77 2e 64  69 67 69 63 65 72 74 2e 63 6f  6d 31 2f 30 2d 06 03 55 04 03 
13 26 44 69 67 69 43 65 72 74  20 53 48 41 32 20 48 69 67 68  20 41 73 73 75 72 61 6e 63 65  20 53 65 72 76 65 72 20 43 41 
30 1e 17 0d 31 36 31 32 30 39  30 30 30 30 30 30 5a 17 0d 31  38 30 31 32 35 31 32 30 30 30  30 5a 30 69 31 0b 30 09 06 03 
55 04 06 13 02 55 53 31 13 30  11 06 03 55 04 08 13 0a 43 61  6c 69 66 6f 72 6e 69 61 31 13  30 11 06 03 55 04 07 13 0a 4d 
65 6e 6c 6f 20 50 61 72 6b 31  17 30 15 06 03 55 04 0a 13 0e  46 61 63 65 62 6f 6f 6b 2c 20  49 6e 63 2e 31 17 30 15 06 03 
55 04 03 0c 0e 2a 2e 66 61 63  65 62 6f 6f 6b 2e 63 6f 6d 30  59 30 13 06 07 2a 86 48 ce 3d  02 01 06 08 2a 86 48 ce 3d 03 
01 07 03 42 00 04 a0 f1 8c af  a7 39 88 68 5b 13 56 0e 15 15  b4 a7 45 ef 1b c7 e5 85 3c 2b  04 d4 65 8a 31 31 22 ea a3 92 
ed 64 9d ba 65 81 e3 b6 12 76  d8 b3 0b 45 f1 ff 0a 28 14 9c  4f dc 73 a9 b3 49 2d a0 76 d3  a3 82 05 4a 30 82 05 46 30 1f 
06 03 55 1d 23 04 18 30 16 80  14 51 68 ff 90 af 02 07 75 3c  cc d9 65 64 62 a2 12 b8 59 72  3b 30 1d 06 03 55 1d 0e 04 16 
04 14 cb 98 28 8c a1 72 e7 e2  32 a9 9a 3d f2 0d a1 81 56 a1  1a 8b 30 81 c7 06 03 55 1d 11  04 81 bf 30 81 bc 82 0e 2a 2e 
66 61 63 65 62 6f 6f 6b 2e 63  6f 6d 82 0e 2a 2e 66 61 63 65  62 6f 6f 6b 2e 6e 65 74 82 08  2a 2e 66 62 2e 63 6f 6d 82 0b 
2a 2e 66 62 63 64 6e 2e 6e 65  74 82 0b 2a 2e 66 62 73 62 78  2e 63 6f 6d 82 10 2a 2e 6d 2e  66 61 63 65 62 6f 6f 6b 2e 63 
6f 6d 82 0f 2a 2e 6d 65 73 73  65 6e 67 65 72 2e 63 6f 6d 82  0e 2a 2e 78 78 2e 66 62 63 64  6e 2e 6e 65 74 82 0e 2a 2e 78 
79 2e 66 62 63 64 6e 2e 6e 65  74 82 0e 2a 2e 78 7a 2e 66 62  63 64 6e 2e 6e 65 74 82 0c 66  61 63 65 62 6f 6f 6b 2e 63 6f 
6d 82 06 66 62 2e 63 6f 6d 82  0d 6d 65 73 73 65 6e 67 65 72  2e 63 6f 6d 30 0e 06 03 55 1d  0f 01 01 ff 04 04 03 02 07 80 
30 1d 06 03 55 1d 25 04 16 30  14 06 08 2b 06 01 05 05 07 03  01 06 08 2b 06 01 05 05 07 03  02 30 75 06 03 55 1d 1f 04 6e 
30 6c 30 34 a0 32 a0 30 86 2e  68 74 74 70 3a 2f 2f 63 72 6c  33 2e 64 69 67 69 63 65 72 74  2e 63 6f 6d 2f 73 68 61 32 2d 
68 61 2d 73 65 72 76 65 72 2d  67 35 2e 63 72 6c 30 34 a0 32  a0 30 86 2e 68 74 74 70 3a 2f  2f 63 72 6c 34 2e 64 69 67 69 
63 65 72 74 2e 63 6f 6d 2f 73  68 61 32 2d 68 61 2d 73 65 72  76 65 72 2d 67 35 2e 63 72 6c  30 4c 06 03 55 1d 20 04 45 30 
43 30 37 06 09 60 86 48 01 86  fd 6c 01 01 30 2a 30 28 06 08  2b 06 01 05 05 07 02 01 16 1c  68 74 74 70 73 3a 2f 2f 77 77 
77 2e 64 69 67 69 63 65 72 74  2e 63 6f 6d 2f 43 50 53 30 08  06 06 67 81 0c 01 02 02 30 81  83 06 08 2b 06 01 05 05 07 01 
01 04 77 30 75 30 24 06 08 2b  06 01 05 05 07 30 01 86 18 68  74 74 70 3a 2f 2f 6f 63 73 70  2e 64 69 67 69 63 65 72 74 2e 
63 6f 6d 30 4d 06 08 2b 06 01  05 05 07 30 02 86 41 68 74 74  70 3a 2f 2f 63 61 63 65 72 74  73 2e 64 69 67 69 63 65 72 74 
2e 63 6f 6d 2f 44 69 67 69 43  65 72 74 53 48 41 32 48 69 67  68 41 73 73 75 72 61 6e 63 65  53 65 72 76 65 72 43 41 2e 63 
72 74 30 0c 06 03 55 1d 13 01  01 ff 04 02 30 00 30 82 02 b0  06 0a 2b 06 01 04 01 d6 79 02  04 02 04 82 02 a0 04 82 02 9c 
02 9a 00 76 00 a4 b9 09 90 b4  18 58 14 87 bb 13 a2 cc 67 70  0a 3c 35 98 04 f9 1b df b8 e3  77 cd 0e c8 0d dc 10 00 00 01 
58 e5 d3 62 04 00 00 04 03 00  47 30 45 02 21 00 ef 58 5b 14  7a a5 84 ff 1c 08 61 c5 cf bd  b6 f9 13 1c df 1c 16 53 37 07 
e7 51 fe 11 3c ef e5 06 02 20  74 e1 a6 af 48 fb fa ef 3f 4b  ee ed 7c 5c 3b d5 91 23 aa 9c  09 10 b9 7f b0 41 6e 41 d3 e7 
a7 04 01 2f 00 ac 3b 9a ed 7f  a9 67 47 57 15 9e 6d 7d 57 56  72 f9 d9 81 00 94 1e 9b de ff  ec a1 31 3b 75 78 2d 00 00 01 
58 e5 d3 62 28 00 00 04 01 01  00 62 f9 cc 57 e0 5f 3f 75 ab  93 8c 85 40 4f d9 c7 95 ac 8d  26 9b 60 ea 4b 8f 54 e3 e7 11 
5a a1 5a 07 74 6a 8b 09 ff c7  8b e5 6f d0 84 72 78 f2 85 04  24 96 d3 ed de 03 9f 3d f9 69  c2 0c cc 1c 26 50 58 79 95 7c 
32 5b c7 15 98 37 6b ae 94 87  1f ce 02 fa 1e b5 bd 26 0a d5  29 9e 62 04 53 02 d7 fd e3 0d  12 f8 e0 da a4 c7 df fd f7 ca 
c0 a8 da 5d b9 fb f1 1c 47 cb  a9 17 ed 8a 21 c9 a7 93 95 b0  68 9c df ab b7 05 3d 07 38 0a  f7 33 99 31 da b0 98 bf 8e 65 
5a 21 a2 7d 1c 2e 74 83 77 71  e7 ce 13 41 7d ba 94 32 dd 74  5f cb 8b 8b 84 6c 9e d3 4a 8a  67 4f 38 d0 75 60 6d 59 b3 05 
87 76 dc 7e 11 42 29 aa 8a da  42 09 57 76 e0 ba 9e 47 89 b1  27 67 62 cf 9b a9 5e 91 8d 3b  d9 91 42 18 ce 10 61 ce 2e 92 
ac e8 bb 62 56 09 19 86 a3 47  4c 1c 98 67 bd 05 e9 9c 84 06  c0 04 ea 08 3e 10 d4 00 76 00  56 14 06 9a 2f d7 c2 ec d3 f5 
e1 bd 44 b2 3e c7 46 76 b9 bc  99 11 5c c0 ef 94 98 55 d6 89  d0 dd 00 00 01 58 e5 d3 62 b9  00 00 04 03 00 47 30 45 02 20 
1a 15 d7 6f 05 0e e5 b0 f8 4e  cf d0 d2 0a 99 c1 44 b6 be 5f  c0 e0 77 a6 93 ba 2f c6 5f 36  bf da 02 21 00 9b b6 47 e5 0d 
76 dc ba 76 38 30 04 02 6c a9  67 7a c3 c6 73 f9 df b4 0f 38  c4 38 49 a7 11 47 59 00 77 00  ee 4b bd b7 75 ce 60 ba e1 42 
69 1f ab e1 9e 66 a3 0f 7e 5f  b0 72 d8 83 00 c4 7b 89 7a a8  fd cb 00 00 01 58 e5 d3 64 11  00 00 04 03 00 48 30 46 02 21 
00 ab 9b 9e 04 28 6b 92 1a 93  c7 82 3a b7 df 1f b5 f1 dc 03  f5 34 c6 f9 a4 5c 37 e6 7b 76  55 27 91 02 21 00 e3 b2 aa d5 
44 1c 0d 72 04 b1 5f 96 9e e3  dd f4 b3 39 92 97 5e bb 4f 93  88 72 ad 4c b4 50 af 4b 30 0d  06 09 2a 86 48 86 f7 0d 01 01 
0b 05 00 03 82 01 01 00 3a cd  4a ca 28 9d b8 f5 60 68 f0 cb  db 7d c0 ff 7e 91 90 70 ac d3  0e da 7e 4d e5 f4 0d 84 66 b0 
90 40 17 fa 55 e5 ce 09 b7 4e  a6 25 4c d8 ba b0 21 5b 3c 10  04 ae 29 b7 68 c5 65 5a e3 a5  72 d8 27 8e d0 1d 99 d5 99 e2 
a1 0e 36 b0 4b 1c f1 a0 ba 40  c9 dd 47 a7 fc 40 bf 83 02 ba  29 d0 42 6f 84 ed a7 a3 0c 3d  8d 5c 85 bf dc 89 32 e2 dd 05 
14 53 88 78 38 95 aa 52 3b dc  70 10 83 b3 f2 f2 74 d8 ca 59  96 53 c4 b2 66 c5 14 a6 7e 9c  82 53 7b 94 d9 71 c7 30 d9 68 
f1 44 c8 96 a6 6c 3a d7 6a 86  0e ff 43 f7 2f 30 31 c3 91 c2  3f b6 a7 a5 9a 0d 0c 0f 61 fd  2a 97 26 02 f5 64 c2 08 e7 f8 
15 69 bf 91 c4 2e 5f b4 10 fd  14 85 89 27 23 50 7a 2c 17 9c  26 e8 fb 8c 8d ba 79 76 5d 5f  7e 42 62 4a 48 fd 26 40 f4 a2 
be 39 82 87 87 ab c2 c4 aa a7  45 9f 59 41 43 a3 5e 72 16 29  07 95 e4 78

The above binary is encoded in DER flavor of ASN.1. We can use the excellent javascript ASN.1 decoder to decode it for us:

30 82 07 e4             ;30=SEQUENCE (0x07e4 bytes)
|- tbsCertificate
|  30 82 06 cc                          ;30=SEQUENCE (0x66cc bytes)
|  |- Version (0x03 bytes)
|  |    a0 03 02 01 02
|  |- SerialNumber (0x10 bytes)
|  |    02 10 0c 00 99 b7 d7 89 c9 f6 66 26 31 7e bc ea 7c 1c
|  |- Signature (0x0d bytes)
|  |    30 0d 06 09 2a 86 48 86 f7 0d 01 01 0b 05 00
|  |- Issuer (0x70 bytes)
|  |    30 70 31 0b 30 09 06 03 55 04 06 13 02 55 53 31 15 30 13 06 03 55 04 0a 13 0c 44 69 67 69 43 65 72 74 20 49 6e 63 31 19 30 17 06 03 55 04 0b 13 10 77 77 77 2e 64 69 67 69 63 65 72 74 2e 63 6f 6d 31 2f 30 2d 06 03 55 04 03 13 26 44 69 67 69 43 65 72 74 20 53 48 41 32 20 48 69 67 68 20 41 73 73 75 72 61 6e 63 65 20 53 65 72 76 65 72 20 43 41 
|  |- Validity (0x1e bytes)
|  |    30 1e 17 0d 31 36 31 32 30 39 30 30 30 30 30 30 5a 17 0d 31 38 30 31 32 35 31 32 30 30 30 30 5a 
|  |- Subject (0x69 bytes)
|  |    30 69 31 0b 30 09 06 03 55 04 06 13 02 55 53 31 13 30 11 06 03 55 04 08 13 0a 43 61 6c 69 66 6f 72 6e 69 61 31 13 30 11 06 03 55 04 07 13 0a 4d 65 6e 6c 6f 20 50 61 72 6b 31 17 30 15 06 03 55 04 0a 13 0e 46 61 63 65 62 6f 6f 6b 2c 20 49 6e 63 2e 31 17 30 15 06 03 55 04 03 0c 0e 2a 2e 66 61 63 65 62 6f 6f 6b 2e 63 6f 6d 
|  |- SubjectPublicKeyInfo (0x59 bytes)
|  |    30 59 
|  |    |- AlgorithmIdentifier (0x13 bytes)
|  |    |  30 13
|  |    |     06 07 2a 86 48 ce 3d 02 01     ;Algorithm Object Identifier - 1.2.840.10045.2.1   ecPublicKey (ANSI X9.62 public key type)
|  |    |     06 08 2a 86 48 ce 3d 03 01 07  ;Algorithm Object Identiifer - 1.2.840.10045.3.1.7 prime256v1  (ANSI X9.62 named elliptic curve)
|  |    |- SubjectPublicKey (0x42 bytes)
|  |       03 42           
|  |          00 04 a0 f1 8c af a7 39 88 68  5b 13 56 0e 15 15 b4 a7 45 ef 
|  |          1b c7 e5 85 3c 2b 04 d4 65 8a  31 31 22 ea a3 92 ed 64 9d ba 
|  |          65 81 e3 b6 12 76 d8 b3 0b 45  f1 ff 0a 28 14 9c 4f dc 73 a9 
|  |          b3 49 2d a0 76 d3
|  |- issuerUniqueID [0]
|       a3 82 05 4a
|          30 82 05 46 30 1f 06 03 55 1d  23 04 18 30 16 80 14 51 68 ff  90 af 02 07 75 3c cc d9 65 64  62 a2 12 b8 59 72 3b 30 1d 06 
|          4b 8f 54 e3 e7 11 5a a1 5a 07 74 6a 8b 09 ff c7 8b e5 6f d0 84 72 78 f2 85 04 24 96 d3 ed de 03 9f 3d f9 69 c2 0c cc 1c 
|          26 50 58 79 95 7c 32 5b c7 15 98 37 6b ae 94 87 1f ce 02 fa 1e b5 bd 26 0a d5 29 9e 62 04 53 02 d7 fd e3 0d 12 f8 e0 da 
|          a4 c7 df fd f7 ca c0 a8 da 5d b9 fb f1 1c 47 cb a9 17 ed 8a 21 c9 a7 93 95 b0 68 9c df ab b7 05 3d 07 38 0a f7 33 99 31 
|          da b0 98 bf 8e 65 5a 21 a2 7d 1c 2e 74 83 77 71 e7 ce 13 41 7d ba 94 32 dd 74 5f cb 8b 8b 84 6c 9e d3 4a 8a 67 4f 38 d0 
|          75 60 6d 59 b3 05 87 76 dc 7e 11 42 29 aa 8a da 42 09 57 76 e0 ba 9e 47 89 b1 27 67 62 cf 9b a9 5e 91 8d 3b d9 91 42 18 
|          ce 10 61 ce 2e 92 ac e8 bb 62 56 09 19 86 a3 47 4c 1c 98 67 bd 05 e9 9c 84 06 c0 04 ea 08 3e 10 d4 00 76 00 56 14 06 9a 
|          2f d7 c2 ec d3 f5 e1 bd 44 b2 3e c7 46 76 b9 bc 99 11 5c c0 ef 94 98 55 d6 89 d0 dd 00 00 01 58 e5 d3 62 b9 00 00 04 03 
|          00 47 30 45 02 20 1a 15 d7 6f 05 0e e5 b0 f8 4e cf d0 d2 0a 99 c1 44 b6 be 5f c0 e0 77 a6 93 ba 2f c6 5f 36 bf da 02 21 
|          00 9b b6 47 e5 0d 76 dc ba 76 38 30 04 02 6c a9 67 7a c3 c6 73 f9 df b4 0f 38 c4 38 49 a7 11 47 59 00 77 00 ee 4b bd b7 
|          75 ce 60 ba e1 42 69 1f ab e1 9e 66 a3 0f 7e 5f b0 72 d8 83 00 c4 7b 89 7a a8 fd cb 00 00 01 58 e5 d3 64 11 00 00 04 03 
|          00 48 30 46 02 21 00 ab 9b 9e 04 28 6b 92 1a 93 c7 82 3a b7 df 1f b5 f1 dc 03 f5 34 c6 f9 a4 5c 37 e6 7b 76 55 27 91 02 
|          21 00 e3 b2 aa d5 44 1c 0d 72 04 b1 5f 96 9e e3 dd f4 b3 39 92 97 5e bb 4f 93 88 72 ad 4c b4 50 af 4b 
|- signatureAlgorithm
|    30 0d 06 09 2a 86 48 86 f7 0d 01 01 0b 05 00 
|- signatureValue
     03 82 01 01
        00 3a cd 4a ca 28 9d b8 f5  60 68 f0 cb db 7d c0 ff  7e 91 90 70 ac d3 0e da  7e 4d e5 f4 0d 84 66 b0 
        90 40 17 fa 55 e5 ce 09 b7  4e a6 25 4c d8 ba b0 21  5b 3c 10 04 ae 29 b7 68  c5 65 5a e3 a5 72 d8 27 
        8e d0 1d 99 d5 99 e2 a1 0e  36 b0 4b 1c f1 a0 ba 40  c9 dd 47 a7 fc 40 bf 83  02 ba 29 d0 42 6f 84 ed 
        a7 a3 0c 3d 8d 5c 85 bf dc  89 32 e2 dd 05 14 53 88  78 38 95 aa 52 3b dc 70  10 83 b3 f2 f2 74 d8 ca 
        59 96 53 c4 b2 66 c5 14 a6  7e 9c 82 53 7b 94 d9 71  c7 30 d9 68 f1 44 c8 96  a6 6c 3a d7 6a 86 0e ff 
        43 f7 2f 30 31 c3 91 c2 3f  b6 a7 a5 9a 0d 0c 0f 61  fd 2a 97 26 02 f5 64 c2  08 e7 f8 15 69 bf 91 c4 
        2e 5f b4 10 fd 14 85 89 27  23 50 7a 2c 17 9c 26 e8  fb 8c 8d ba 79 76 5d 5f  7e 42 62 4a 48 fd 26 40 
        f4 a2 be 39 82 87 87 ab c2  c4 aa a7 45 9f 59 41 43  a3 5e 72 16 29 07 95 e4  78

Which is a lot

But the only part we care about is the SubjectPublicKeyInfo:

SubjectPublicKeyInfo (0x59 bytes)
|    30 59 
|    |- AlgorithmIdentifier (0x13 bytes)
|    |  30 13
|    |     06 07 2a 86 48 ce 3d 02 01     ;Algorithm Object Identifier - 1.2.840.10045.2.1   ecPublicKey (ANSI X9.62 public key type)
|    |     06 08 2a 86 48 ce 3d 03 01 07  ;Algorithm Object Identiifer - 1.2.840.10045.3.1.7 prime256v1  (ANSI X9.62 named elliptic curve)
|    |- SubjectPublicKey (0x42 bytes)
|       03 42           
|          00 04 a0 f1 8c af a7 39 88 68  5b 13 56 0e 15 15 b4 a7 45 ef 
|          1b c7 e5 85 3c 2b 04 d4 65 8a  31 31 22 ea a3 92 ed 64 9d ba 
|          65 81 e3 b6 12 76 d8 b3 0b 45  f1 ff 0a 28 14 9c 4f dc 73 a9 
|          b3 49 2d a0 76 d3

If you take just those bytes:

30 59 30 13 06 07 2a 86 48 ce  3d 02 01 06 08 2a 86 48 ce 3d
03 01 07 03 42 00 04 a0 f1 8c  af a7 39 88 68 5b 13 56 0e 15 
15 b4 a7 45 ef 1b c7 e5 85 3c  2b 04 d4 65 8a 31 31 22 ea a3 
92 ed 64 9d ba 65 81 e3 b6 12  76 d8 b3 0b 45 f1 ff 0a 28 14 
9c 4f dc 73 a9 b3 49 2d a0 76  d3

And run them through SHA256, you get:

• SHA256: 854206f3b721ef511941885904492ad952812e386eb2c530ee747cc32b98eeb6 (hex)

You then base64 encode that, and you get:

• base64: hUIG87ch71EZQYhZBEkq2VKBLjhussUw7nR8wyuY7rY=

That is the hash that you add to your HTTP response headers:

Public-Key-Pins: max-age=31536000; pin-sha256="hUIG87ch71EZQYhZBEkq2VKBLjhussUw7nR8wyuY7rY=";

Cross-check

Our calculated values matches the PKP hash that SSL Labs returns:

You can also paste the original PEM (base64) encoded certificate into https://certpins.appspot.com/pin and confirm that it returns the same PKP hash:

Public-Key-Pins: max-age=31536000;  pin-sha256="hUIG87ch71EZQYhZBEkq2VKBLjhussUw7nR8wyuY7rY=";

Why SubjectPublicKeyInfo?

A public key is a number (or in the case of RSA a pair of numbers):

• exponent (e.g. 65,537)
• modulus (e.g. one hundred and fifty four million googol googol googol)

Over the years there are a number of ways of storing these two numbers:

SHH:

[lenghPrefix]ssh-rsa[lengthPrefix][exponent][lengthPrefix][modulus]

Xml

<RSAKeyValue>
   <Modulus>ANxn+vSe8nIdRSy0gHkGoJQnUIIJ3WfOV7hsSk9An9LRafuZXYUMB6H5RxtWFm72f7nPKlg2N5kpqk+oEuhPx4IrnXIqnN5vwu4Sbc/w8rjE3XxcGsgXUams3wgiBJ0r1/lLCd6a61xRGtj4+Vae+Ps3mz/TdGUkDf80dVek9b9V</Modulus>
   <Exponent>AQAB</Exponent>
</RSAKeyValue>

PKCS#1

-----BEGIN RSA PUBLIC KEY-----
MIGJAoGBANxn+vSe8nIdRSy0gHkGoJQnUIIJ3WfOV7hsSk9An9LRafuZXY
UMB6H5RxtWFm72f7nPKlg2N5kpqk+oEuhPx4IrnXIqnN5vwu4Sbc/w8rjE
3XxcGsgXUams3wgiBJ0r1/lLCd6a61xRGtj4+Vae+Ps3mz/TdGUkDf80dV
ek9b9VAgMBAAE=
-----END RSA PUBLIC KEY-----

PKCS#1 chose to use the DER flavor of ASN.1 encoding of the following structure:

RSAPublicKey ::= SEQUENCE {
      modulus           INTEGER,  -- n
      publicExponent    INTEGER   -- e
  }

Later when new public key encryption systems came along, they "borrowed" the RSAPublicKey structure, but prefixed it with an Algorithm ID:

SubjectPublicKeyInfo  ::=  SEQUENCE  {
    algorithm            AlgorithmIdentifier,
    subjectPublicKey     RSAPublicKey }

Where AlgorithID for PKCS#1 RSAPublicKey is 1.2.840.113549.1.1.1, which comes from:

• 1 - ISO assigned OIDs
  • 1.2 - ISO member body
    • 1.2.840 - USA
      • 1.2.840.113549 - RSADSI
        • 1.2.840.113549.1 - PKCS
          • 1.2.840.113549.1.1 - PKCS-1

This format would then appear in base64 encoded files as:

-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDcZ/r0nvJyHUUstIB5BqCUJ1CC
Cd1nzle4bEpPQJ/S0Wn7mV2FDAeh+UcbVhZu9n+5zypYNjeZKapPqBLoT8eCK51y
Kpzeb8LuEm3P8PK4xN18XBrIF1GprN8IIgSdK9f5SwnemutcURrY+PlWnvj7N5s/
03RlJA3/NHVXpPW/VQIDAQAB
-----END PUBLIC KEY-----

This is the SubjectPublicKeyInfo that you get the sha256 hash of.

Inside every certificate is a SubjectPublicKeyInfo; even a Windows EFS certificate.

You can use OpenSSL to:

• extract the SubjectPublicKeyInfo chunk and save it as a BEGIN PUBLIC KEY file
• hash it
• base64 it

People need to stop treating OpenSSL as a magical crypto box. Too often, people just type in esoteric openssl commands to do something, rather than understanding what they're doing. Even the very RFC on Public Key Pinning doesn't bother to explain what it is, and just tells you to go to the all-knowing openssl. OpenSSL is not the be-all and end-off of encryption management.

Metheglin

Good explanation, thanks. May i know the reason why OpenSSL is unfriendly certificate manipulation utility ? – Patmos

@Patmos It has no user interface. – Metheglin

Thanks! they should document it! – Mesocarp

Hi @IanBoyd, Can you please look at my question : #59311230 and tell me what i am doing wrong .. Do i need to pin public key ? If yes how do i do it ? There are no proper information online on how to pin public key .. plz help – Drusy

Getting a Subject Key Identifier is answered at security.stackexchange.com/a/128956/158091 – Berglund

The RFC does define pins and SPKI fingerprints, of course: rfc-editor.org/rfc/rfc7469.html#section-2.4 – Luminal

Answer 3

There is a really straight forward way of doing this. I spent some time scraping things together on this one to produce a very reusable solution. Unfortunately, I have only done this on OSX, but you should be able to follow along fairly easy if you're on another OS.
For this method you are going to need:

• Install Homebrew

• OpenSSL (brew install openssl in terminal after installing Homebrew)

• Create new .sh file and set contents to be:

  openssl s_client -servername $1 -connect $1:443 | openssl x509 -pubkey -noout | openssl rsa -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64

• Save file to disk

• From terminal window, call bash <new sh file directory> www.google.com

This should now return the hash that you require and leave you with a nice little reusable solution. You may have to press ctrl+c after the hash is returned.

It should leave you with something like the following:
Hope this helps someone out.
Thanks

Au

TrustKit provides a Python script to do this : github.com/datatheorem/TrustKit/blob/master/… – Batton

@Batton Yes, but this requires that you have a .pem or .der file on your computer, which isn't always possible/ creates a lot more work. My solution helps if you're trying to pin someone else's cert easily. – Au

amazing! for ubuntu you can install it using sudo apt install linuxbrew-wrapper command – Rhebarhee

where can I find backup key ? – Oeuvre

@ChangdeoJadhav What backup key? If it's a separate question, then create a new question on SO so it gets more attention. – Au

This seems like a great and easy answer. Just one question - I got a different result for google.com 47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU= I thought the public key is static and never changes? – Vraisemblance

still helping people in 2021. to avoid the <kbd>ctrl</kbd>+<kbd>c</kbd> part I had to redirect to output to /dev/null though openssl s_client -servername $1 -connect $1:443 < /dev/null | openssl x509 -pubkey -noout | openssl rsa -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64 – Dahle

When running the script from @Climbatize, I got an error: 140302128756032:error:0607907F:digital envelope routines:EVP_PKEY_get0_RSA:expecting an rsa key:../crypto/evp/p_lib.c:469 So I changed openssl rsa into pkey and it works OK now: openssl s_client -servername $1 -connect $1:443 < /dev/null | openssl x509 -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64 – Volding

Answer 4

Assuming you have openssl and the certificate file, you can use this command taken from here.

openssl x509 -in my-certificate.crt -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64

Metaphysics