In Rust, rcgen generates a CSR in a few lines: KeyPair::generate_for creates the P-256 key, CertificateParams::new takes the SANs, and serialize_request signs the PKCS#10 request with ECDSA and SHA-256. The run writes key.pem and csr.pem and decodes the CSR with OpenSSL.

Cargo.toml — rcgen with its default ring backend

1[dependencies]
2rcgen = "=0.14.10" # default features: ring backend and PEM output

src/main.rs — P-256 key and CSR with two SANs

1use rcgen::{CertificateParams, DistinguishedName, DnType, KeyPair, PKCS_ECDSA_P256_SHA256};
2 
3fn main() -> Result<(), Box<dyn std::error::Error>> {
4    let key = KeyPair::generate_for(&PKCS_ECDSA_P256_SHA256)?;
5 
6    // new() takes the SANs (DNS names or IP addresses)
7    let mut params = CertificateParams::new(vec!["api.example.com".to_owned(), "www.example.com".to_owned()])?;
8    // Replace the default subject ("CN=rcgen self signed cert")
9    let mut subject = DistinguishedName::new();
10    subject.push(DnType::CountryName, "US");
11    subject.push(DnType::OrganizationName, "Example Corp");
12    subject.push(DnType::CommonName, "api.example.com");
13    params.distinguished_name = subject;
14 
15    let csr = params.serialize_request(&key)?;
16    std::fs::write("csr.pem", csr.pem()?)?;
17    std::fs::write("key.pem", key.serialize_pem())?; // PKCS#8
18    Ok(())
19}

How it works

KeyPair::generate_for(&PKCS_ECDSA_P256_SHA256) creates the key; the algorithm constant also fixes the signature algorithm.
CertificateParams::new(vec![…]) turns each string into a DNS SAN, or an IP SAN if it parses as an IP address.
DistinguishedName::new() plus push builds the subject in the order given.
serialize_request(&key) returns the CSR; csr.pem() and key.serialize_pem() write the PEM files.

Gotchas

CertificateParams starts with the subject CN=rcgen self signed cert. Forgetting to replace distinguished_name sends that name to the CA.
CertificateParams also carries validity, key usages and CA flags that a CSR does not transmit; for a CSR only the subject, the SANs and the extensions requested matter.
rcgen's default features pull in ring; on platforms where ring does not build, use default-features = false with the aws_lc_rs and pem features.
serialize_request is not deterministic — two calls give different signatures for the same input — so do not compare CSR bytes in tests.

How to generate a CSR for an EC P-256 key with SANs in Rust

How it works

Gotchas

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