first commit, i i have no idea what i have done

server/internal/crypto/argon2id.go

@@ -0,0 +1,102 @@
+package crypto
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"golang.org/x/crypto/argon2"
+)
+
+type Argon2Params struct {
+	Memory  uint32 // KiB
+	Time    uint32
+	Threads uint8
+	SaltLen uint32
+	KeyLen  uint32
+}
+
+var DefaultArgon2 = Argon2Params{
+	Memory:  7168, // 7 MiB
+	Time:    5,
+	Threads: 1,
+	SaltLen: 16,
+	KeyLen:  32,
+}
+
+// Hash returns PHC string: $argon2id$v=19$m=...,t=...,p=...$<salt>$<hash>
+func Hash(password string, p Argon2Params) (string, error) {
+	salt := make([]byte, p.SaltLen)
+	if _, err := rand.Read(salt); err != nil {
+		return "", err
+	}
+	sum := argon2.IDKey([]byte(password), salt, p.Time, p.Memory, p.Threads, p.KeyLen)
+	return fmt.Sprintf("$argon2id$v=19$m=%d,t=%d,p=%d$%s$%s",
+		p.Memory, p.Time, p.Threads,
+		base64.RawStdEncoding.EncodeToString(salt),
+		base64.RawStdEncoding.EncodeToString(sum),
+	), nil
+}
+
+func Verify(password, phc string) (bool, error) {
+	parts := strings.Split(phc, "$")
+	// Expect: ["", "argon2id", "v=19", "m=...,t=...,p=...", "<saltB64>", "<hashB64>"]
+	if len(parts) != 6 || parts[1] != "argon2id" || parts[2] != "v=19" {
+		return false, errors.New("invalid PHC header")
+	}
+
+	// parse params
+	var mem, iters uint64
+	var threads uint64
+	for _, kv := range strings.Split(parts[3], ",") {
+		if strings.HasPrefix(kv, "m=") {
+			v := strings.TrimPrefix(kv, "m=")
+			x, err := strconv.ParseUint(v, 10, 32)
+			if err != nil {
+				return false, fmt.Errorf("mem: %w", err)
+			}
+			mem = x
+		} else if strings.HasPrefix(kv, "t=") {
+			v := strings.TrimPrefix(kv, "t=")
+			x, err := strconv.ParseUint(v, 10, 32)
+			if err != nil {
+				return false, fmt.Errorf("time: %w", err)
+			}
+			iters = x
+		} else if strings.HasPrefix(kv, "p=") {
+			v := strings.TrimPrefix(kv, "p=")
+			x, err := strconv.ParseUint(v, 10, 8)
+			if err != nil {
+				return false, fmt.Errorf("threads: %w", err)
+			}
+			threads = x
+		}
+	}
+	if mem == 0 || iters == 0 || threads == 0 {
+		return false, errors.New("invalid PHC params")
+	}
+
+	salt, err := b64DecodeFlex(parts[4])
+	if err != nil {
+		return false, fmt.Errorf("salt decode: %w", err)
+	}
+
+	want, err := b64DecodeFlex(parts[5])
+	if err != nil {
+		return false, fmt.Errorf("hash decode: %w", err)
+	}
+
+	got := argon2.IDKey([]byte(password), salt, uint32(iters), uint32(mem), uint8(threads), uint32(len(want)))
+	return subtle.ConstantTimeCompare(got, want) == 1, nil
+}
+
+func b64DecodeFlex(s string) ([]byte, error) {
+	if b, err := base64.RawStdEncoding.DecodeString(s); err == nil {
+		return b, nil
+	}
+	return base64.StdEncoding.DecodeString(s) // padded fallback
+}