How to extract the public key from an X.509 certificate in Rust

1[dependencies]
2x509-parser = "=0.18.1"
3pem-rfc7468 = { version = "=1.0.0", features = ["alloc"] }
4sha2 = "=0.11.1"
5base64 = "=0.23.1"
6 

1use base64::Engine;
2use base64::engine::general_purpose::STANDARD;
3use sha2::{Digest, Sha256};
4use x509_parser::pem::parse_x509_pem;
5 
6fn main() -> Result<(), Box<dyn std::error::Error>> {
7    let data = std::fs::read("cert.pem")?;
8    let (_, pem) = parse_x509_pem(&data)?;
9    let cert = pem.parse_x509()?;
10 
11    // .raw: the SubjectPublicKeyInfo DER exactly as the certificate stores it
12    let spki = cert.public_key().raw;
13    let spki_pem = pem_rfc7468::encode_string("PUBLIC KEY", pem_rfc7468::LineEnding::LF, spki)?;
14    std::fs::write("pub.pem", &spki_pem)?;
15    print!("{spki_pem}");
16 
17    let digest = Sha256::digest(spki);
18    let hex: Vec<String> = digest.iter().map(|b| format!("{b:02x}")).collect();
19    println!("SPKI SHA-256: {}", hex.join(":"));
20    println!("SPKI pin (base64): {}", STANDARD.encode(digest));
21 
22    // subject_public_key is only the BIT STRING inside: the bare EC point, not an SPKI
23    let point = &cert.public_key().subject_public_key.data;
24    println!("subject_public_key.data: {} bytes, starts with {:02x}", point.len(), point[0]);
25    Ok(())
26}
27