x509cert is a tool and library for generating X.509 certificates and certificate requests. It is written in C99 and uses BearSSL to decode keys and compute signatures.
Like BearSSL, libx509cert makes minimal use of the standard library. It does not do any dynamic allocation, and uses only a few basic libc functions.
x509cert [-C] [-a altname]... [-c issuercert] [-k issuerkey] [-b notbefore] [-d duration] [-s serial] key [subject]
x509cert -r [-a altname]... key [subject]
If -r is used, a PKCS#10 CertificateRequest is created. Otherwise,
an X.509 Certificate is created. In either case, the structure is
PEM-encoded and written to standard output. For more details, see
x509cert(1).
See x509cert(3).