p256 signature using cryptokit fails verification on openssl

Question

I've went through all the posts with similar info about signature or keys used not working with openssll. But I haven't been able to patch it all together. I will use some sample keys for what I tried,

let privPem = """
-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgIUSrwhllMSminPZZ
Gx0YHUsL12IWIGI+4yhejpq90HihRANCAAT6pxKtIKm4VbfXeKpQ7rxITlC6b18Q
0X+Iz1UVDolyjx79bt5vUp0mPJ6hHBnK/Ap5gXpv89wmLPp7/O2NconE
-----END PRIVATE KEY-----
"""

let privKey = try! P256.Signing.PrivateKey(pemRepresentation: privPem)

let pubKey = privKey.publicKey

let challengeDev = "1122334455667788"

let dataToSignDev = challengeDev.hexadecimal

let digest = SHA256.hash(data: dataToSignDev!)
let signatureForDigest = try! privKey.signature(
    for: digest)


let signature1 = try! P256.Signing.ECDSASignature(derRepresentation: signatureForDigest.derRepresentation)

let isValidSignature = pubKey.isValidSignature(signatureForDigest, for: digest)

I have tried the same using Security framework also to no avail. What I tried is a direct application of what the openssl part does, So I have a device that will verify the signature that the iOS app will be sending. The public key is taken from a certificate I would share with the device. All data sent to the device including signature is in DER format.


openssl dgst -sha256 -sign app_private.key -out %OUT_RESOURCES_DIR%\signature.der -binary device_challenge.hex

openssl x509 -inform der -in cert.der -out cert.pem
openssl x509 -pubkey -noout -in cert.pem > public_key.pem
openssl dgst -sha256 -verify public_key.pem -signature signature.der challenge.hex

Here on the iOS side If I were to sign and verify everything is fine. But if the same signature is verified on OpenSSL it fails.

I tried to create a DER file on the terminal but asn1parse fails on it,

Test % echo 30450220198944e2a8352941036f227225940392cbd1bc720358ce29db29a2a85f2b2a30022100b4e75ceb0335e4b1955aab01edc8e7347f78dc627f8d02a78103cd9165571d57 > signature1.der

Test % openssl asn1parse -inform DER -in signature1.der
    0:d=0  hl=2 l=  48 cons: PRINTABLESTRING   
Error in encoding
140704639042368:error:0DFFF09B:asn1 encoding routines:CRYPTO_internal:too long:/AppleInternal/Library/BuildRoots/97f6331a-ba75-11ed-a4bc-863efbbaf80d/Library/Caches/com.apple.xbs/Sources/libressl/libressl-3.3/crypto/asn1/asn1_lib.c:143:

I'm assuming I need to manually do some changes to make them inter compatible? Like this post Can't export EC kSecAttrTokenIDSecureEnclave public key

Not sure how to get there though. All help appreciated.

Apple CryptoKit

716

Posted by

Jz120

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@eskimo please check

—
Jz120

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Answer 1

This is working for me. To start, I built this program with Xcode 14.3:

import Foundation
import CryptoKit

let privateKeyPEM = """
-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgIUSrwhllMSminPZZ
Gx0YHUsL12IWIGI+4yhejpq90HihRANCAAT6pxKtIKm4VbfXeKpQ7rxITlC6b18Q
0X+Iz1UVDolyjx79bt5vUp0mPJ6hHBnK/Ap5gXpv89wmLPp7/O2NconE
-----END PRIVATE KEY-----
"""

let message = Data("Hello Cruel World!\n".utf8)

func main() {
    let privateKey = try! P256.Signing.PrivateKey(pemRepresentation: privateKeyPEM)
    let publicKey = privateKey.publicKey
    let digest = SHA256.hash(data: message)
    let signature = try! privateKey.signature(for: digest)
    print("public key:")
    print(publicKey.pemRepresentation)
    print("signature:")
    print((signature.derRepresentation as NSData).debugDescription)
}

main()

Running on macOS 13.5 it prints:

public key:
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE+qcSrSCpuFW313iqUO68SE5Qum9f
ENF/iM9VFQ6Jco8e/W7eb1KdJjyeoRwZyvwKeYF6b/PcJiz6e/ztjXKJxA==
-----END PUBLIC KEY-----
signature:
<30450221 00aa0c2e 0d56cac8 c8eec31c bb066ee0 565d8355 fb6ff133 6e537601 f79fd69f fd022048 fb636022 f613fba5 07a15c46 c4de88a1 583418dc 4d49c55d 02422f99 b7e428>

IMPORTANT You’ll get a different signature each time because X9.62 signatures are non-deterministic.

I then transferred that to the shell:

% cat public_key.pem                                                         
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE+qcSrSCpuFW313iqUO68SE5Qum9f
ENF/iM9VFQ6Jco8e/W7eb1KdJjyeoRwZyvwKeYF6b/PcJiz6e/ztjXKJxA==
-----END PUBLIC KEY-----
% xxd signature.dat     
00000000: 3045 0221 00aa 0c2e 0d56 cac8 c8ee c31c  0E.!.....V......
00000010: bb06 6ee0 565d 8355 fb6f f133 6e53 7601  ..n.V].U.o.3nSv.
00000020: f79f d69f fd02 2048 fb63 6022 f613 fba5  ...... H.c`"....
00000030: 07a1 5c46 c4de 88a1 5834 18dc 4d49 c55d  ..\F....X4..MI.]
00000040: 0242 2f99 b7e4 28                        .B/...(
% xxd message.txt
00000000: 4865 6c6c 6f20 4372 7565 6c20 576f 726c  Hello Cruel Worl
00000010: 6421 0a                                  d!.

It verifies as expected:

% openssl dgst -sha256 -verify public_key.pem -signature signature.dat message.txt
Verified OK

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Posted by

eskimo

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Answer 2

Thanks for checking @eskimo , you're right it's working.

Posted by

Jz120

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p256 signature using cryptokit fails verification on openssl

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