Certificate Signing Requests Explained

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I regularly help developers with code signing problems and I find that a lot of those problems stem from a fundamental misunderstanding of how code signing requests work. This post is my attempt at explaining that.

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

Certificate Signing Requests Explained

I see a lot of folks confused by how code signing requests (CSRs) work, and that causes problems down the line. This is my attempt to explain the process and head off those problems.

IMPORTANT This post covers the ‘classic’ certificate creation process described in Developer Account Help > Create certificates > Create a certificate signing request. Things work differently if you use Xcode to create your certificates, and very differently if you use cloud-managed certificates.

Here’s a basic outline of the CSR process:

  1. You run Keychain Access and choose Certificate Assistant > Request a Certificate from a Certificate Authority.

  2. You run through the workflow as described in Developer Account Help > Create certificates > Create a certificate signing request.

  3. This does two things:

    • It generates a public / private key pair in your keychain. To see these, run Keychain Access and select “login” on the left and Keys at the top. Look for keys whose names match the Common Name you entered in step 2.

    • It prompts you to save a .certSigningRequest file (CSR). This contains a copy of the public key.

  4. You upload the CSR file to the developer web site.

  5. The developer web site issues you a certificate. In human terms this certificate says “Apple certifies that the subject of this certificate holds the private key that matches the public key embedded in this certificate.”

    Note The developer web site sets the subject information in the certificate based on your developer account. It ignores the subject information in the CSR. So, you can enter any information you want in step 2. This is a good way to distinguish between different keys in your keychain. For example, you might set the Common Name field in step 2 to include a unique identifier that allows you to easily identify the public / private key pair generated in step 3.

  6. You download the certificate and add it to your keychain.

At this point your keychain contains a digital identity, that is, a certificate and the private key that matches the public key embedded in that certificate. To see this in Keychain Access, select “login” on the left and My Certificates at the top.

What’s This My Certificates Thing?

There’s an industry-wide terminology problem here. Folks use the term certificate to mean two different things:

  • A digital identity, that is, a certificate and its matching private key

  • An actual certificate

This industry-wide confusion extends into the Apple ecosystem. For example:

  • The Security framework gets this right, drawing a clear distinction between a digital identity (SecIdentity) and a certificate (SecCertificate).

  • Keychain Access uses My Certificates for digital identities.

  • Other user-facing apps use different terms. For example, Apple Configurator uses signing identity (yay for them!). OTOH, the help for Apple Mail uses the term personal certificate.

  • Xcode and its documentation uses the term signing certificate to denote a digital identity that can be used for code signing.

This terminological inexactitude causes all sorts of problems. For example, imagine you’re setting up a new Mac. You download your certificate from the developer web site and then wonder why you can’t sign your code. That’s because the developer web site gives you a certificate, not a digital identity. Indeed, the developer web site can’t give you a digital identity because it never got a copy of your private key [1].

[1] Again, we’re talking about the classic certificate creation process here; this statement is not true for cloud-managed certificates.

Digital Identity Formation

Apple platforms form a digital identity by:

  1. Extracting the public key from the certificate.

  2. Calculating a SHA-1 digest of that.

  3. Looking for a private key whose kSecAttrApplicationLabel attribute matches that SHA-1 hash.

For more background on this, see my SecItem attributes for keys post.

Note that it’s perfectly valid for multiple certificates to match against the same private key, yielding a digital identity for each certificate. You regularly see this when you renew a certificate.

Looking Inside a CSR

A CSR is a PEM file (PEM is short for Privacy-Enhanced Mail) with the CERTIFICATE REQUEST label:

% cat CertificateSigningRequest.certSigningRequest 
-----BEGIN CERTIFICATE REQUEST-----
MIICgjCCAWoCAQAwPTEcMBoGCSqGSIb3DQEJARYNZnJvZ0Bmcm9nLmNvbTEQMA4G
…
Ur9x5voYb6CafUBZMMiYw6aFXcgnsx4ZXxe8VEqNCarrQi+9tqitDD/bCuymT5Da
2+t64DGjpVM2lwtwqvH6Qh6QdPjkUw==
-----END CERTIFICATE REQUEST-----

To see inside, run the openssl tool as shown below:

% openssl req -in CertificateSigningRequest.certSigningRequest -text -noout
Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject: emailAddress=mrgumby@opendoor.com, CN=Mr Gumby, C=US
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
                    00:b1:b4:a0:15:4d:4a:d7:29:1d:ed:d6:b7:c2:7c:
                    …
                    28:b9:8a:58:a4:04:63:fe:45:b2:4f:db:bd:93:20:
                    4e:8b
                Exponent: 65537 (0x10001)
        Attributes:
            a0:00
    Signature Algorithm: sha256WithRSAEncryption
         80:f9:0e:73:8e:42:d8:3c:e3:e0:06:54:13:d7:48:ef:a8:71:
         …
         2f:74:e1:2e:cf:e7:ed:3e:64:b4:78:85:f4:ac:38:07:b1:15:
         6b:3c:39:f9

For even more details, convert the file to DER form and then dump that as ASN.1:

% openssl req -in CertificateSigningRequest.certSigningRequest -out CertificateSigningRequest.der -outform der
% dumpasn1 -p -a CertificateSigningRequest.der
SEQUENCE {
  SEQUENCE {
    INTEGER 0
    SEQUENCE {
      SET {
        SEQUENCE {
          OBJECT IDENTIFIER emailAddress (1 2 840 113549 1 9 1)
          IA5String 'mrgumby@opendoor.com'
          }
        }
      SET {
        SEQUENCE {
          OBJECT IDENTIFIER commonName (2 5 4 3)
          UTF8String 'Mr Gumby'
          }
        }
      SET {
        SEQUENCE {
          OBJECT IDENTIFIER countryName (2 5 4 6)
          PrintableString 'US'
          }
        }
      }
    SEQUENCE {
      SEQUENCE {
        OBJECT IDENTIFIER rsaEncryption (1 2 840 113549 1 1 1)
        NULL
        }
      BIT STRING, encapsulates {
        SEQUENCE {
          INTEGER
            00 B1 B4 A0 15 4D 4A D7 29 1D ED D6 B7 C2 7C 74
            …
            28 B9 8A 58 A4 04 63 FE 45 B2 4F DB BD 93 20 4E
            8B
          INTEGER 65537
          }
        }
      }
    [0]
      Error: Object has zero length.
    }
  SEQUENCE {
    OBJECT IDENTIFIER sha256WithRSAEncryption (1 2 840 113549 1 1 11)
    NULL
    }
  BIT STRING
    80 F9 0E 73 8E 42 D8 3C E3 E0 06 54 13 D7 48 EF
    …
    ED 3E 64 B4 78 85 F4 AC 38 07 B1 15 6B 3C 39 F9
  }

I’m using the dumpasn1 tool, available here.

To extract the public key from the CSR, run this command:

% openssl req -in CertificateSigningRequest.certSigningRequest -noout -pubkey -out public.pem
% cat public.pem 
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAsbSgFU1K1ykd7da3wnx0
…
FymGqUEcwfIISlG1C9VXyMRPzDcMrzjHm4i9qI9NliYouYpYpARj/kWyT9u9kyBO
iwIDAQAB
-----END PUBLIC KEY-----

To further explore that key, use the techniques in my On Cryptographic Key Formats post.

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