Pull request: 4925-refactor-tls-vol-1

Merge in DNS/adguard-home from 4925-refactor-tls-vol-1 to master

Squashed commit of the following:

commit ad87b2e93183b28f2e38666cc4267fa8dfd1cca0
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Oct 14 18:49:22 2022 +0300

    all: refactor tls, vol. 1

    Co-Authored-By: Rahul Somasundaram <Rahul.Somasundaram@checkpt.com>

internal/aghtls/aghtls.go

@@ -1,7 +1,48 @@
 // Package aghtls contains utilities for work with TLS.
 package aghtls
 
-import "crypto/tls"
+import (
+	"crypto/tls"
+	"fmt"
+
+	"github.com/AdguardTeam/golibs/log"
+)
+
+// init makes sure that the cipher name map is filled.
+//
+// TODO(a.garipov): Propose a similar API to crypto/tls.
+func init() {
+	suites := tls.CipherSuites()
+	cipherSuites = make(map[string]uint16, len(suites))
+	for _, s := range suites {
+		cipherSuites[s.Name] = s.ID
+	}
+
+	log.Debug("tls: known ciphers: %q", cipherSuites)
+}
+
+// cipherSuites are a name-to-ID mapping of cipher suites from crypto/tls.  It
+// is filled by init.  It must not be modified.
+var cipherSuites map[string]uint16
+
+// ParseCiphers parses a slice of cipher suites from cipher names.
+func ParseCiphers(cipherNames []string) (cipherIDs []uint16, err error) {
+	if cipherNames == nil {
+		return nil, nil
+	}
+
+	cipherIDs = make([]uint16, 0, len(cipherNames))
+	for _, name := range cipherNames {
+		id, ok := cipherSuites[name]
+		if !ok {
+			return nil, fmt.Errorf("unknown cipher %q", name)
+		}
+
+		cipherIDs = append(cipherIDs, id)
+	}
+
+	return cipherIDs, nil
+}
 
 // SaferCipherSuites returns a set of default cipher suites with vulnerable and
 // weak cipher suites removed.