diff --git a/cmd/hydroxide/hydroxide.go b/cmd/hydroxide/hydroxide.go
new file mode 100644
index 0000000..1e21b2d
--- /dev/null
+++ b/cmd/hydroxide/hydroxide.go
@@ -0,0 +1,38 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/emersion/hydroxide/protonmail"
+)
+
+func main() {
+	c := &protonmail.Client{
+		RootURL: "https://dev.protonmail.com/api",
+		AppVersion: "Web_3.11.1",
+		ClientID: "Web",
+		ClientSecret: "4957cc9a2e0a2a49d02475c9d013478d",
+	}
+
+	scanner := bufio.NewScanner(os.Stdin)
+
+	fmt.Printf("Username: ")
+	scanner.Scan()
+	username := scanner.Text()
+
+	fmt.Printf("Password: ")
+	scanner.Scan()
+	password := scanner.Text()
+
+	fmt.Printf("2FA code: ")
+	scanner.Scan()
+	code := scanner.Text()
+
+	err := c.Auth(username, password, code, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
diff --git a/protonmail/auth.go b/protonmail/auth.go
new file mode 100644
index 0000000..c425158
--- /dev/null
+++ b/protonmail/auth.go
@@ -0,0 +1,137 @@
+package protonmail
+
+import (
+	"encoding/base64"
+	"net/http"
+
+	"log"
+)
+
+type authInfoReq struct {
+	ClientID string
+	ClientSecret string
+	Username string
+}
+
+type AuthInfoResp struct {
+	resp
+	Version int
+	TwoFactor int
+	Modulus string
+	ServerEphemeral string
+	Salt string
+	SRPSession string
+}
+
+type AuthInfo struct {
+	TwoFactor int
+	version int
+	modulus string
+	serverEphemeral string
+	salt string
+	srpSession string
+}
+
+func (c *Client) AuthInfo(username string) (*AuthInfo, error) {
+	reqData := &authInfoReq{
+		ClientID: c.ClientID,
+		ClientSecret: c.ClientSecret,
+		Username: username,
+	}
+
+	req, err := c.newJSONRequest(http.MethodPost, "/auth/info", reqData)
+	if err != nil {
+		return nil, err
+	}
+
+	var respData AuthInfoResp
+	if err := c.doJSON(req, &respData); err != nil {
+		return nil, err
+	}
+
+	return &AuthInfo{
+		TwoFactor: respData.TwoFactor,
+		version: respData.Version,
+		modulus: respData.Modulus,
+		serverEphemeral: respData.ServerEphemeral,
+		salt: respData.Salt,
+		srpSession: respData.SRPSession,
+	}, nil
+}
+
+type authReq struct {
+	ClientID string
+	ClientSecret string
+	Username string
+	SRPSession string
+	ClientEphemeral string
+	ClientProof string
+	TwoFactorCode string
+}
+
+type PasswordMode int
+
+const (
+	PasswordSingle PasswordMode = 1
+	PasswordTwo = 2
+)
+
+type authResp struct {
+	resp
+	AccessToken string
+	ExpiresIn int
+	TokenType string
+	Scope string
+	UID string `json:"Uid"`
+	RefreshToken string
+	EventID string
+	ServerProof string
+	PasswordMode PasswordMode
+	PrivateKey string
+	KeySalt string
+}
+
+func (c *Client) Auth(username, password, twoFactorCode string, info *AuthInfo) error {
+	if info == nil {
+		var err error
+		if info, err = c.AuthInfo(username); err != nil {
+			return err
+		}
+	}
+
+	log.Printf("%#v\n", info)
+
+	proofs, err := srp([]byte(password), info)
+	if err != nil {
+		return err
+	}
+
+	reqData := &authReq{
+		ClientID: c.ClientID,
+		ClientSecret: c.ClientSecret,
+		Username: username,
+		SRPSession: info.srpSession,
+		ClientEphemeral: base64.StdEncoding.EncodeToString(proofs.clientEphemeral),
+		ClientProof: base64.StdEncoding.EncodeToString(proofs.clientProof),
+		TwoFactorCode: twoFactorCode,
+	}
+	log.Printf("%#v\n", reqData)
+
+	req, err := c.newJSONRequest(http.MethodPost, "/auth", reqData)
+	if err != nil {
+		return err
+	}
+
+	var respData authResp
+	if err := c.doJSON(req, &respData); err != nil {
+		return err
+	}
+
+	log.Printf("%#v\n", respData)
+
+	if err := proofs.VerifyServerProof(respData.ServerProof); err != nil {
+		return err
+	}
+
+	return nil
+}
diff --git a/protonmail/protonmail.go b/protonmail/protonmail.go
new file mode 100644
index 0000000..cdf9854
--- /dev/null
+++ b/protonmail/protonmail.go
@@ -0,0 +1,94 @@
+// Package protonmail implements a ProtonMail API client.
+package protonmail
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"net/http"
+	"strconv"
+)
+
+const Version = 2
+
+const headerAPIVersion = "X-Pm-Apiversion"
+
+type resp struct {
+	Code int
+	*apiError
+}
+
+func (r *resp) Err() error {
+	if err := r.apiError; err != nil {
+		return r.apiError
+	}
+	return nil
+}
+
+type maybeError interface {
+	Err() error
+}
+
+type apiError struct {
+	Message string `json:"Error"`
+}
+
+func (err apiError) Error() string {
+	return err.Message
+}
+
+// Client is a ProtonMail API client.
+type Client struct {
+	HTTPClient *http.Client
+	RootURL string
+	AppVersion string
+	ClientID string
+	ClientSecret string
+}
+
+func (c *Client) newRequest(method, path string, body io.Reader) (*http.Request, error) {
+	req, err := http.NewRequest(method, c.RootURL + path, body)
+	if err != nil {
+		return nil, err
+	}
+
+	req.Header.Set("X-Pm-Appversion", c.AppVersion)
+	req.Header.Set(headerAPIVersion, strconv.Itoa(Version))
+
+	return req, nil
+}
+
+func (c *Client) newJSONRequest(method, path string, body interface{}) (*http.Request, error) {
+	var b bytes.Buffer
+	if err := json.NewEncoder(&b).Encode(body); err != nil {
+		return nil, err
+	}
+	return c.newRequest(method, path, &b)
+}
+
+func (c *Client) do(req *http.Request) (*http.Response, error) {
+	httpClient := c.HTTPClient
+	if httpClient == nil {
+		httpClient = http.DefaultClient
+	}
+	return httpClient.Do(req)
+}
+
+func (c *Client) doJSON(req *http.Request, respData interface{}) error {
+	resp, err := c.do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	if err := json.NewDecoder(resp.Body).Decode(respData); err != nil {
+		return err
+	}
+
+	if maybeError, ok := respData.(maybeError); ok {
+		if err := maybeError.Err(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
diff --git a/protonmail/srp.go b/protonmail/srp.go
new file mode 100644
index 0000000..eddd775
--- /dev/null
+++ b/protonmail/srp.go
@@ -0,0 +1,215 @@
+package protonmail
+
+import (
+	"bytes"
+	"crypto/rand"
+	"crypto/sha512"
+	"crypto/subtle"
+	"encoding/base64"
+	"errors"
+	"io"
+	"math/big"
+
+	"github.com/emersion/go-bcrypt"
+	"golang.org/x/crypto/openpgp"
+	"golang.org/x/crypto/openpgp/clearsign"
+	openpgperrors "golang.org/x/crypto/openpgp/errors"
+)
+
+var randReader io.Reader = rand.Reader
+
+func decodeModulus(msg string) ([]byte, error) {
+	block, _ := clearsign.Decode([]byte(msg))
+	if block == nil {
+		return nil, errors.New("invalid modulus signed PGP block")
+	}
+
+	// TODO: check signature key
+	_, err := openpgp.CheckDetachedSignature(nil, bytes.NewReader(block.Plaintext), block.ArmoredSignature.Body)
+	if err != nil && err != openpgperrors.ErrUnknownIssuer {
+		return nil, err
+	}
+
+	return base64.StdEncoding.DecodeString(string(block.Plaintext))
+}
+
+func expandHash(b []byte) []byte {
+	var expanded []byte
+	var part [64]byte
+
+	part = sha512.Sum512(append(b, 0))
+	expanded = append(expanded, part[:]...)
+
+	part = sha512.Sum512(append(b, 1))
+	expanded = append(expanded, part[:]...)
+
+	part = sha512.Sum512(append(b, 2))
+	expanded = append(expanded, part[:]...)
+
+	part = sha512.Sum512(append(b, 3))
+	expanded = append(expanded, part[:]...)
+
+	return expanded
+}
+
+func hashPassword(version int, password, salt, modulus []byte) ([]byte, error) {
+	switch version {
+	case 3, 4:
+		salt = append(salt, []byte("proton")...)
+		hashed, err := bcrypt.GenerateFromPasswordAndSalt(password, 10, salt)
+		if err != nil {
+			return nil, err
+		}
+
+		hashed = bytes.Replace(hashed, []byte("$2a$"), []byte("$2y$"), 1)
+		return expandHash(append([]byte(hashed), modulus...)), nil
+	default:
+		return nil, errors.New("unsupported auth version")
+	}
+}
+
+func reverse(b []byte) {
+	for i := 0; i < len(b)/2; i++ {
+		j := len(b) - 1 - i
+		b[i], b[j] = b[j], b[i]
+	}
+}
+
+func itoa(i *big.Int, l int) []byte {
+	b := i.Bytes()
+	reverse(b)
+	padding := make([]byte, l/8 - len(b))
+	b = append(b, padding...)
+	return b
+}
+
+func atoi(b []byte) *big.Int {
+	reverse(b)
+	return big.NewInt(0).SetBytes(b)
+}
+
+type proofs struct {
+	clientEphemeral []byte
+	clientProof []byte
+	expectedServerProof []byte
+}
+
+// From https://github.com/ProtonMail/WebClient/blob/public/src/app/authentication/services/srp.js#L13
+func generateProofs(l int, hash func([]byte) []byte, modulusBytes, hashedBytes, serverEphemeralBytes []byte) (*proofs, error) {
+	generator := big.NewInt(2)
+
+	multiplier := atoi(hash(append(itoa(generator, l), modulusBytes...)))
+	modulus := atoi(modulusBytes)
+	hashed := atoi(hashedBytes)
+	serverEphemeral := atoi(serverEphemeralBytes)
+
+	modulusMinusOne := big.NewInt(0).Sub(modulus, big.NewInt(1))
+	if modulus.BitLen() != l {
+		return nil, errors.New("SRP modulus has incorrect size")
+	}
+
+	multiplier = multiplier.Mod(multiplier, modulus)
+
+	if multiplier.Cmp(big.NewInt(1)) <= 0 || multiplier.Cmp(modulusMinusOne) >= 0 {
+		return nil, errors.New("SRP multiplier is out of bounds")
+	}
+	if generator.Cmp(big.NewInt(1)) <= 0 || generator.Cmp(modulusMinusOne) >= 0 {
+		return nil, errors.New("SRP generator is out of bounds")
+	}
+	if serverEphemeral.Cmp(big.NewInt(1)) <= 0 || serverEphemeral.Cmp(modulusMinusOne) >= 0 {
+		return nil, errors.New("SRP server ephemeral is out of bounds")
+	}
+
+	// TODO: Check primality
+	// TODO: Check safe primality
+
+	var clientSecret, clientEphemeral, scramblingParam *big.Int
+	for {
+		for {
+			var err error
+			clientSecret, err = rand.Int(randReader, modulusMinusOne)
+			if err != nil {
+				return nil, err
+			}
+
+			if clientSecret.Cmp(big.NewInt(int64(l)*2)) <= 0 { // Very unlikely
+				continue
+			}
+			break
+		}
+
+		clientEphemeral = big.NewInt(0).Exp(generator, clientSecret, modulus)
+		scramblingParam = atoi(hash(append(itoa(clientEphemeral, l), itoa(serverEphemeral, l)...)))
+		if scramblingParam.Cmp(big.NewInt(0)) == 0 { // Very unlikely
+			continue
+		}
+		break
+	}
+
+	subtracted := big.NewInt(0).Sub(serverEphemeral, big.NewInt(0).Mod(big.NewInt(0).Mul(big.NewInt(0).Exp(generator, hashed, modulus), multiplier), modulus))
+	if subtracted.Cmp(big.NewInt(0)) < 0 {
+		subtracted.Add(subtracted, modulus)
+	}
+	exponent := big.NewInt(0).Mod(big.NewInt(0).Add(big.NewInt(0).Mul(scramblingParam, hashed), clientSecret), modulusMinusOne)
+	sharedSession := big.NewInt(0).Exp(subtracted, exponent, modulus)
+
+	var clientProof []byte
+	clientProof = append(clientProof, itoa(clientEphemeral, l)...)
+	clientProof = append(clientProof, itoa(serverEphemeral, l)...)
+	clientProof = append(clientProof, itoa(sharedSession, l)...)
+	clientProof = hash(clientProof)
+
+	var serverProof []byte
+	serverProof = append(serverProof, itoa(clientEphemeral, l)...)
+	serverProof = append(serverProof, clientProof...)
+	serverProof = append(serverProof, itoa(sharedSession, l)...)
+	serverProof = hash(serverProof)
+
+	return &proofs{
+		clientEphemeral: itoa(clientEphemeral, l),
+		clientProof: clientProof,
+		expectedServerProof: serverProof,
+	}, nil
+}
+
+func (p *proofs) VerifyServerProof(serverProofString string) error {
+	serverProof, err := base64.StdEncoding.DecodeString(serverProofString)
+	if err != nil {
+		return err
+	}
+
+	if subtle.ConstantTimeCompare(p.expectedServerProof, serverProof) != 1 {
+		return errors.New("invalid server proof")
+	}
+	return nil
+}
+
+// From https://github.com/ProtonMail/WebClient/blob/public/src/app/authentication/services/srp.js#L135
+func srp(password []byte, info *AuthInfo) (*proofs, error) {
+	modulus, err := decodeModulus(info.modulus)
+	if err != nil {
+		return nil, err
+	}
+
+	serverEphemeral, err := base64.StdEncoding.DecodeString(info.serverEphemeral)
+	if err != nil {
+		return nil, err
+	}
+
+	salt, err := base64.StdEncoding.DecodeString(info.salt)
+	if err != nil {
+		return nil, err
+	}
+
+	hashed, err := hashPassword(info.version, password, salt, modulus)
+	if err != nil {
+		return nil, err
+	}
+
+	proofs, err := generateProofs(2048, expandHash, modulus, hashed, serverEphemeral)
+	if err != nil {
+		return nil, err
+	}
+
+	return proofs, nil
+}