Add MLDSA-44 cosigner implementation (#247)

Adds an implementation of the ML-DSA-44 cosigner recently added to the tlog-cosignature spec.

This signer can be used as a note Signer for checkpoint messages, or to sign Subtrees directly.

Author: AlCutter

go.mod

@@ -3,6 +3,8 @@ module github.com/transparency-dev/formats
 go 1.25.0
 
 require (
+	filippo.io/mldsa v0.0.0-20260215214346-43d0283efc3e
 	github.com/google/go-cmp v0.7.0
+	golang.org/x/crypto v0.50.0
 	golang.org/x/mod v0.35.0
 )

go.sum

@@ -1,4 +1,8 @@
+filippo.io/mldsa v0.0.0-20260215214346-43d0283efc3e h1:VsUbObBMxXlc23Eb9VeeJYE4jvTs87qa5RqSN2U5FJU=
+filippo.io/mldsa v0.0.0-20260215214346-43d0283efc3e/go.mod h1:32qQ5yj3R24Eu03iWFWchdC3OB653wPvoepWejkefbY=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
+golang.org/x/crypto v0.50.0 h1:zO47/JPrL6vsNkINmLoo/PH1gcxpls50DNogFvB5ZGI=
+golang.org/x/crypto v0.50.0/go.mod h1:3muZ7vA7PBCE6xgPX7nkzzjiUq87kRItoJQM1Yo8S+Q=
 golang.org/x/mod v0.35.0 h1:Ww1D637e6Pg+Zb2KrWfHQUnH2dQRLBQyAtpr/haaJeM=
 golang.org/x/mod v0.35.0/go.mod h1:+GwiRhIInF8wPm+4AoT6L0FA1QWAad3OMdTRx4tFYlU=