Stupid certificate tricks

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Sometimes I do things for no real reasons other than "because I can" and/or "it amuses me". For example, embedding a snarky message into my HTTPS certificate.

Before I get into how this works, take a moment to admire the results:

$ openssl s_client -connect rya.nc:443 2>/dev/null</dev/null | sed -n /BEGIN/,/END/p | egrep --color '^//.*|$'
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

In case you're wondering, "bayl bhgynjf jvyy unir cevinpl" is "only outlaws will have privacy" run through ROT13.

This is a the PEM (Base64) formatted representation of the certificate. Decoded, it represents the following:

Certificate:
Data:
Version: 3 (0x2)
Serial Number: 1415621 (0x1599c5)
Signature Algorithm: sha1WithRSAEncryption
Issuer: C=US, O=GeoTrust, Inc., CN=RapidSSL CA
Validity
Not Before: Oct 6 04:32:55 2014 GMT
Not After : Oct 11 09:43:04 2015 GMT
Subject: serialNumber=ig/OAsn1rJnAwys5Dp/H6-AxS2/42l8-,
OU=GT88999859, OU=See www.rapidssl.com/resources/cps (c)14,
OU=Domain Control Validated - RapidSSL(R), CN=www.rya.nc
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
Public-Key: (2048 bit)
Modulus:
00:a6:28:97:df:03:4a:8b:25:64:a6:3c:f7:86:55:
60:69:c4:e9:9f:90:ee:ea:af:09:c0:40:82:21:47:
77:8f:71:6c:d7:25:e8:ff:f5:a1:7a:7f:dc:af:2a:
6d:a2:0a:da:a6:1c:bf:8a:cf:e8:ba:d9:5a:c1:e7:
7f:6d:ac:a5:fd:b8:60:ca:78:df:fe:3b:f2:cb:fb:
a7:8a:bf:dc:7a:f8:a7:a6:5f:ff:ff:ab:b9:ea:a4:
5e:dd:fb:ea:55:bf:44:91:d6:6a:b6:0b:58:bf:a6:
a4:0b:9b:39:b7:70:68:3c:5c:ab:33:75:04:32:e8:
2b:01:7e:ea:e8:41:8f:97:89:36:91:68:33:1e:12:
ea:e0:8a:d1:10:d1:a2:95:e6:96:b8:ba:e4:b8:42:
32:ab:06:ad:b6:23:3c:0c:69:bd:b6:a3:a5:9d:66:
79:cc:14:5c:12:97:ef:89:92:8b:46:b7:50:9d:c4:
92:66:ab:5f:5c:0e:c7:58:12:0f:96:e2:c3:21:48:
0c:d5:84:65:7e:28:93:bc:04:c0:2f:b0:eb:a8:da:
96:28:89:6c:ab:b2:59:7b:a8:e7:0a:4b:91:26:84:
76:6b:40:40:82:e0:c4:ba:5f:e4:b0:5d:f9:d6:34:
c7:3d:ce:69:16:25:46:78:52:f5:dc:3e:ef:0c:ed:
0a:a7
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Authority Key Identifier:
keyid:6B:69:3D:6A:18:42:4A:DD:8F:02:65:39:FD:35:24:86:78:91:16:30

X509v3 Key Usage: critical
Digital Signature, Key Encipherment
X509v3 Extended Key Usage:
TLS Web Server Authentication, TLS Web Client Authentication
X509v3 Subject Alternative Name:
DNS:www.rya.nc, DNS:rya.nc
X509v3 CRL Distribution Points:

Full Name:
URI:http://rapidssl-crl.geotrust.com/crls/rapidssl.crl

X509v3 Subject Key Identifier:
83:F2:2A:5E:A0:9A:13:7D:D9:BC:91:AE:06:EE:BC:53:74:53:EA:7E
X509v3 Basic Constraints: critical
CA:FALSE
Authority Information Access:
OCSP - URI:http://rapidssl-ocsp.geotrust.com
CA Issuers - URI:http://rapidssl-aia.geotrust.com/rapidssl.crt

X509v3 Certificate Policies:
Policy: 2.16.840.1.113733.1.7.54
CPS: http://www.geotrust.com/resources/cps

Signature Algorithm: sha1WithRSAEncryption
60:2c:5c:e4:b0:ff:33:bb:73:e7:8a:71:12:f9:6e:60:75:ff:
a5:e2:f4:bb:b2:9a:29:13:4f:ad:55:eb:02:a0:15:b2:ee:28:
78:ee:9e:81:78:1a:ce:a3:ab:20:24:b3:08:89:20:76:8e:01:
84:13:9e:94:b2:a3:f8:62:60:ad:65:08:fa:9d:0d:59:99:0b:
d7:38:60:39:d6:70:f8:05:5a:a4:31:24:b9:7b:76:ed:68:20:
59:7c:95:8d:1f:17:3c:37:f7:65:73:fa:e7:5f:91:b6:19:3e:
a4:c7:dd:a1:f5:fc:49:63:b2:58:f1:60:f7:82:b1:05:8a:86:
f7:fb:f8:5d:0c:6c:10:16:3c:0f:f9:b8:64:70:d8:d6:79:d4:
c1:6b:27:af:b3:5f:89:90:13:65:1b:36:49:5e:bb:c9:56:06:
70:5c:f8:90:af:80:63:98:1c:15:8a:5b:84:33:27:32:b9:c1:
0f:bb:6a:5b:0d:98:2a:46:e7:52:b9:e2:72:11:ae:26:50:e0:
f1:1f:1f:77:12:16:17:cc:bc:5a:74:fc:83:b9:15:df:b9:eb:
61:db:d5:00:88:69:c7:ba:24:79:ac:80:92:a0:4b:55:f5:18:
f6:82:bd:db:c4:c7:45:37:52:92:ba:81:a4:4b:aa:1f:a8:fa:
81:e4:f5:d6

The section of the modulus containing the message is highlighted. The finer points of RSA are beyond the scope of this post, but it's important to know that the modulus, n, is the product of two secret prime numbers of the same length, p and q. Normally they would be chosen at random, but in this case that's been tweaked.

import gmpy
from Crypto.PublicKey import RSA
from binascii import hexlify, unhexlify
from base64 import b64encode as b64e, b64decode as b64d

def replace_at(orig, replace, offset):
    return orig[0:offset] + replace + orig[offset+len(replace):]

msg_b64 = '//When/cryptography/is/outlawed/bayl/bhgynjf/jvyy/unir/cevinpl//'
msg_bytes = b64d(msg_b64)

# key parameters
keybits, e = 2048, 65537

while True:
    # generate initial random key
    rsa = RSA.generate(keybits)
    # get modulus as bytes
    n_bytes = unhexlify(str(hex(rsa.n))[2:-1])
    p = rsa.p

    # splice in the message, may need to play
    # with the offset for it to come out right
    n_tmp_bytes = replace_at(n_bytes, msg_bytes, 36)

    # convert the modulus bytes back into an integer
    n_tmp = gmpy.mpz(hexlify(n_tmp_bytes), 16)

    # value to start search for a new prime q from
    q_tmp = n_tmp / p
    q_new = q_tmp.next_prime()

    # recompute key components based on new q
    n_new = p * q_new
    phi = (p-1) * (q_new-1)
    # let the loop repeat until phi is coprime to e, then compute d
    if gmpy.gcd(e, phi) == 1:
        d = gmpy.invert(e, phi)
        rsa = RSA.construct((long(n_new), long(e), long(d), long(p), long(q_new)))
        print rsa.exportKey()
        break

The original p is kept, n_tmp is created by splicing the encoded message into n, and q_tmp = n_tmp÷p. The problem is that q_tmp is vanishingly unlikely to be an integer, let alone a prime number. This is resolved by finding q_new, the first prime number larger than q_tmp, using the next_prime function from Python's gmpy library. When n_new = p⋅q_new is computed, it will differ from n_tmp by p⋅(q_new − q_tmp). Since the gaps between prime numbers are relatively small even for very large numbers, only the last half of the digits (give or take a few) of the modulus will be disturbed by the correction, leaving the message intact so long as it wasn't too long. If you're confused as to why this is so, think back to the multiplication method you probably learned as a child.

Running the code gives an RSA private key:

A CSR (Certificate Signing Request) can be generated from the private key like this:

$ openssl req -new -key tmp.key -subj '/C=US/CN=www.example.com'
-----BEGIN CERTIFICATE REQUEST-----
MIICbDCCAVQCAQAwJzELMAkGA1UEBhMCVVMxGDAWBgNVBAMMD3d3dy5leGFtcGxl
LmNvbTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAPCcjl3YkQlTmZqB
BfZNNddNCEu7dlHIvTq7FxujW7NkQEFwcP/1oXp/3K8qbaIK2qYcv4rP6LrZWsHn
f22spf24YMp43/478sv7p4q/3Hr4p6Zf/w/RfeoIbRfwE+TfAQmqHQyr+KAJLh/V
cZfwb2ApIwTEWNy7qzwo0X8pR00ER9GYLsC97vELj9uMU28fJLkQU+JnUcSw+kBM
CWhfjm2G8OUocRCaYBZl6ot8KBiB0Aj9WKfsJkjB20uUiBg5znSwFmzH1WQviBP8
9NFUj65dEflbdSwczTC/TJZdVKLdlJZ+RsAUnCbVKakq/Q3ot3de9MvRmVtCVQ81
XvuFxJsCAwEAAaAAMA0GCSqGSIb3DQEBBQUAA4IBAQA42dokgfBLUVXLw5hqicwe
vTvvZZqizWCjHLpqiOGLOJzQ4DU2OcNadae4KDNZpJ2jEMRUK+zaG91Qq+++YN/E
9dkMDZ10DQQuisyZelidDP/ppifNmFxJzRQnyHEFrM5JfxBHxZIBNJ/PXxTSQfxS
qgNj/2W4PHd0kvMPv9DU/xqmy4eVkwfruPzdx6Om2+45w9pii4rUsPhHBZ3NTJFG
lWCl8SJHkWtwCB2ClGZTETMz/FcxiM1r9PTpTiWD3oZ7ldOJ2IoJ0HB8MqEQUDBT
9K0hvpDxvMFOsY0m7DlAGb0PFv62esnLDYct0r8AgWlBioh/Sk9PlAFvew496sv5
-----END CERTIFICATE REQUEST-----

Notice that the message is no longer visible. With base64 every three bytes are represented by four characters so a byte may be encoded differently depending on its position within its grouping. The additional fields in the CSR changed the offset of the modulus, but the CA (Certificate Authority) will add more fields when signing it, so I'll worry about it once I know what the CA will add.

Still no message. Fortunately many CAs offer free re-issues. After regenerating the key and CSR a few times with different offsets, 59 turns out to be a winner.

So, is this safe to use? I couldn't come up with any attacks that this key generation method introduces, but I didn't try very hard. I certainly don't recommend using it in a commercial setting.

Ryan Castellucci's blog

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Stupid certificate tricks

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