| 1 | //! Certificate Signing Requests, and the key pairs behind them. |
| 2 | //! |
| 3 | //! A CSR is the only thing you ever send Apple when asking for a signing |
| 4 | //! certificate. It carries the *public* half of a freshly generated key |
| 5 | //! pair plus a subject name, self-signed to prove you hold the matching |
| 6 | //! private key. Apple checks that signature, then issues a certificate |
| 7 | //! binding your public key to your team identity. |
| 8 | //! |
| 9 | //! The private half never leaves this process except through |
| 10 | //! [`CertificateRequest::private_key_pem`], and it is the part that actually matters: |
| 11 | //! a certificate whose private key is lost is dead, permanently, because |
| 12 | //! Apple never had a copy to give back. |
| 13 | //! |
| 14 | //! # Why RSA 2048 |
| 15 | //! |
| 16 | //! Apple's certificate service accepts RSA 2048 only. An ECDSA CSR, which |
| 17 | //! is what most modern tooling produces by default, is rejected at |
| 18 | //! submission. That single constraint dictates the whole implementation |
| 19 | //! here: [`rcgen`] cannot generate RSA keys, so the key comes from the |
| 20 | //! [`rsa`] crate and is handed to rcgen as an externally supplied key |
| 21 | //! pair purely for the CSR's self-signature. |
| 22 | |
| 23 | use rcgen::{CertificateParams, DistinguishedName, DnType, KeyPair as RcgenKeyPair}; |
| 24 | use rsa::pkcs8::{EncodePrivateKey, LineEnding}; |
| 25 | use rsa::RsaPrivateKey; |
| 26 | use smbcloud_ascapi_core::{Error, Result}; |
| 27 | use zeroize::Zeroizing; |
| 28 | |
| 29 | /// Apple rejects anything else. Not a tunable. |
| 30 | const RSA_MODULUS_BITS: usize = 2048; |
| 31 | |
| 32 | /// A freshly generated RSA key pair and the CSR that asks Apple to certify |
| 33 | /// it. |
| 34 | /// |
| 35 | /// Keep these two together. The CSR is worthless without the key, the |
| 36 | /// certificate Apple returns is worthless without the key, and the key is |
| 37 | /// unrecoverable if dropped. |
| 38 | pub struct CertificateRequest { |
| 39 | /// PEM-encoded PKCS#10, ready for |
| 40 | /// [`CertificateCreateAttributes::csr_content`](crate::certificate::CertificateCreateAttributes::csr_content). |
| 41 | csr_pem: String, |
| 42 | /// PEM-encoded PKCS#8. Wrapped in [`Zeroizing`] so the bytes are |
| 43 | /// wiped when dropped rather than left in freed heap for whatever |
| 44 | /// allocates next. |
| 45 | private_key_pem: Zeroizing<String>, |
| 46 | } |
| 47 | |
| 48 | impl CertificateRequest { |
| 49 | /// The CSR to send Apple, as PEM text including the |
| 50 | /// `-----BEGIN CERTIFICATE REQUEST-----` armour. |
| 51 | pub fn csr_pem(&self) -> &str { |
| 52 | &self.csr_pem |
| 53 | } |
| 54 | |
| 55 | /// The private key, PKCS#8 PEM. This is the secret: write it somewhere |
| 56 | /// encrypted, and never log it, print it to stdout, or return it from |
| 57 | /// an API a language model can read. |
| 58 | pub fn private_key_pem(&self) -> &Zeroizing<String> { |
| 59 | &self.private_key_pem |
| 60 | } |
| 61 | } |
| 62 | |
| 63 | /// Deliberately opaque: printing a key pair, even by accident through a |
| 64 | /// derived `Debug` on some struct three layers up, is how private keys end |
| 65 | /// up in CI logs. |
| 66 | impl std::fmt::Debug for CertificateRequest { |
| 67 | fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { |
| 68 | f.debug_struct("CertificateRequest") |
| 69 | .field("csr_pem", &"<csr>") |
| 70 | .field("private_key_pem", &"<redacted>") |
| 71 | .finish() |
| 72 | } |
| 73 | } |
| 74 | |
| 75 | /// Generate an RSA 2048 key pair and a PKCS#10 CSR over it. |
| 76 | /// |
| 77 | /// `common_name` becomes the CSR subject's CN. Apple overwrites the |
| 78 | /// subject on the certificate it issues (with `Apple Distribution: Team |
| 79 | /// Name (TEAMID)` or similar), so this value is not what you will see in |
| 80 | /// your keychain afterwards. It is still worth setting to something |
| 81 | /// identifying, because the CSR itself may be archived. |
| 82 | /// |
| 83 | /// Generation is CPU-bound and takes a noticeable moment: finding two |
| 84 | /// 1024-bit primes is not free. Call it off the async executor if you are |
| 85 | /// in one. |
| 86 | pub fn generate_certificate_request(common_name: &str) -> Result<CertificateRequest> { |
| 87 | let mut rng = rand::thread_rng(); |
| 88 | |
| 89 | let private_key = RsaPrivateKey::new(&mut rng, RSA_MODULUS_BITS) |
| 90 | .map_err(|source| Error::KeyGeneration(source.to_string()))?; |
| 91 | |
| 92 | let private_key_pem = private_key |
| 93 | .to_pkcs8_pem(LineEnding::LF) |
| 94 | .map_err(|source| Error::KeyGeneration(source.to_string()))?; |
| 95 | |
| 96 | // rcgen re-parses the PEM rather than taking the rsa crate's type |
| 97 | // directly: the two crates share no key representation, and PKCS#8 PEM |
| 98 | // is the interchange format both agree on. |
| 99 | let rcgen_key = RcgenKeyPair::from_pem(&private_key_pem) |
| 100 | .map_err(|source| Error::CsrGeneration(source.to_string()))?; |
| 101 | |
| 102 | let mut params = CertificateParams::default(); |
| 103 | let mut dn = DistinguishedName::new(); |
| 104 | dn.push(DnType::CommonName, common_name); |
| 105 | params.distinguished_name = dn; |
| 106 | |
| 107 | let csr_pem = params |
| 108 | .serialize_request(&rcgen_key) |
| 109 | .map_err(|source| Error::CsrGeneration(source.to_string()))? |
| 110 | .pem() |
| 111 | .map_err(|source| Error::CsrGeneration(source.to_string()))?; |
| 112 | |
| 113 | Ok(CertificateRequest { |
| 114 | csr_pem, |
| 115 | private_key_pem, |
| 116 | }) |
| 117 | } |
| 118 | |
| 119 | #[cfg(test)] |
| 120 | mod tests { |
| 121 | use super::*; |
| 122 | use std::io::Write; |
| 123 | use std::process::{Command, Stdio}; |
| 124 | |
| 125 | /// Run `openssl` over a PEM on stdin and return stdout **and** stderr |
| 126 | /// joined, or `None` if openssl isn't on this machine. |
| 127 | /// |
| 128 | /// Both streams, deliberately: macOS ships LibreSSL, which prints the |
| 129 | /// `verify OK` banner to stderr while OpenSSL proper puts it on |
| 130 | /// stdout. Reading only one stream makes this test pass or fail based |
| 131 | /// on which implementation the machine happens to have. |
| 132 | fn openssl(args: &[&str], stdin_pem: &str) -> Option<String> { |
| 133 | let mut child = Command::new("openssl") |
| 134 | .args(args) |
| 135 | .stdin(Stdio::piped()) |
| 136 | .stdout(Stdio::piped()) |
| 137 | .stderr(Stdio::piped()) |
| 138 | .spawn() |
| 139 | .ok()?; |
| 140 | child |
| 141 | .stdin |
| 142 | .as_mut() |
| 143 | .expect("stdin piped") |
| 144 | .write_all(stdin_pem.as_bytes()) |
| 145 | .expect("writes"); |
| 146 | let out = child.wait_with_output().expect("waits"); |
| 147 | assert!( |
| 148 | out.status.success(), |
| 149 | "openssl {args:?} failed: {}", |
| 150 | String::from_utf8_lossy(&out.stderr) |
| 151 | ); |
| 152 | Some(format!( |
| 153 | "{}{}", |
| 154 | String::from_utf8_lossy(&out.stdout), |
| 155 | String::from_utf8_lossy(&out.stderr) |
| 156 | )) |
| 157 | } |
| 158 | |
| 159 | #[test] |
| 160 | fn csr_is_rsa_2048_and_self_signature_verifies() { |
| 161 | let request = generate_certificate_request("smbCloud signing test").expect("generates"); |
| 162 | |
| 163 | // Structural checks that hold with or without openssl present. |
| 164 | assert!(request |
| 165 | .csr_pem() |
| 166 | .starts_with("-----BEGIN CERTIFICATE REQUEST-----")); |
| 167 | assert!(request |
| 168 | .private_key_pem() |
| 169 | .starts_with("-----BEGIN PRIVATE KEY-----")); |
| 170 | |
| 171 | // The parts that actually decide whether Apple accepts this: the |
| 172 | // key must be RSA 2048, and the CSR's own signature must verify. |
| 173 | // Asserting on our own library's output would be circular, so this |
| 174 | // asks a completely independent implementation. |
| 175 | let Some(text) = openssl(&["req", "-noout", "-text", "-verify"], request.csr_pem()) else { |
| 176 | eprintln!("openssl not available; skipped the cryptographic assertions"); |
| 177 | return; |
| 178 | }; |
| 179 | |
| 180 | assert!( |
| 181 | text.contains("Public-Key: (2048 bit)"), |
| 182 | "expected an RSA 2048 key, got:\n{text}" |
| 183 | ); |
| 184 | assert!( |
| 185 | text.contains("rsaEncryption"), |
| 186 | "expected an RSA key; Apple rejects ECDSA CSRs. Got:\n{text}" |
| 187 | ); |
| 188 | // openssl prints this only when the CSR's self-signature checks |
| 189 | // out, which is the proof we hold the matching private key. |
| 190 | assert!( |
| 191 | text.contains("Certificate request self-signature verify OK") |
| 192 | || text.contains("verify OK"), |
| 193 | "CSR self-signature did not verify:\n{text}" |
| 194 | ); |
| 195 | assert!( |
| 196 | text.contains("smbCloud signing test"), |
| 197 | "common name missing from subject:\n{text}" |
| 198 | ); |
| 199 | } |
| 200 | |
| 201 | #[test] |
| 202 | fn each_request_gets_a_fresh_key() { |
| 203 | // Reusing a key across certificates means one compromise revokes |
| 204 | // everything, and it is an easy mistake to make when adding |
| 205 | // caching later. |
| 206 | let a = generate_certificate_request("a").expect("generates"); |
| 207 | let b = generate_certificate_request("b").expect("generates"); |
| 208 | assert_ne!( |
| 209 | a.private_key_pem().as_str(), |
| 210 | b.private_key_pem().as_str(), |
| 211 | "two requests shared a private key" |
| 212 | ); |
| 213 | } |
| 214 | |
| 215 | #[test] |
| 216 | fn debug_never_prints_key_material() { |
| 217 | let request = generate_certificate_request("redaction check").expect("generates"); |
| 218 | let rendered = format!("{request:?}"); |
| 219 | assert!(rendered.contains("<redacted>")); |
| 220 | assert!( |
| 221 | !rendered.contains("BEGIN PRIVATE KEY"), |
| 222 | "Debug leaked the private key" |
| 223 | ); |
| 224 | } |
| 225 | } |