diff --git a/app/src/apdu_handler.c b/app/src/apdu_handler.c
index 02f42b3c..4d4286c1 100644
--- a/app/src/apdu_handler.c
+++ b/app/src/apdu_handler.c
@@ -177,8 +177,27 @@ __Z_INLINE void handleInitMASPTransfer(volatile uint32_t *flags,
 
 }
 
+__Z_INLINE void handleExtractSpendDataMASPTransfer(volatile uint32_t *tx, uint32_t rx) {
+    zemu_log("----[handleExtractSpendDataMASPTransfer]\n");
 
-// Get the sapling full viewing key (ak, nk, ovk)
+    *tx = 0;
+    if (rx != APDU_MIN_LENGTH || G_io_apdu_buffer[OFFSET_DATA_LEN] != 0) {
+        THROW(APDU_CODE_COMMAND_NOT_ALLOWED);
+    }
+    // TODO implement the following
+    zxerr_t err = crypto_extract_spend_proof_key_and_rnd(G_io_apdu_buffer, IO_APDU_BUFFER_SIZE - 2);
+    view_tx_state();
+    if (err == zxerr_ok) {
+        *tx = 128; //SPEND_EXTRACT_LEN
+        THROW(APDU_CODE_OK);
+    } else {
+        *tx = 0;
+        THROW(APDU_CODE_DATA_INVALID);
+    }
+
+}
+
+// Get the sapling full viewing key fvk = (ak, nk, ovk, dk)
 __Z_INLINE void handleGetKeyFVK(volatile uint32_t *flags,
                                 volatile uint32_t *tx, uint32_t rx) {
     zemu_log("----[handleGetKeyFVK]\n");
@@ -310,6 +329,7 @@ void handleApdu(volatile uint32_t *flags, volatile uint32_t *tx, uint32_t rx) {
                 }
 
                 // MASP transactions
+                // Get full viewing key fvk = (ak, nk, ovk, dk)
                 case INS_GET_FVK: {
                     zemu_log("----[INS_GET_FVK]\n");
                     CHECK_PIN_VALIDATED()
@@ -317,6 +337,10 @@ void handleApdu(volatile uint32_t *flags, volatile uint32_t *tx, uint32_t rx) {
                     break;
                 }
 
+                // Step 1 in signing a MASP transaction:
+                // the ledger receives an initial transaction blob
+                // and stores relevant information in flash memory,
+                // so that it can check consistency with what it signs later.
                 case INS_INIT_MASP_TRANSFER: {
                     zemu_log("----[INS_INIT_MASP_TRANSFER]\n");
                     CHECK_PIN_VALIDATED()
@@ -324,6 +348,17 @@ void handleApdu(volatile uint32_t *flags, volatile uint32_t *tx, uint32_t rx) {
                     break;
                 }
 
+                // If there are any spends (= shielded inputs) this is
+                // Step 2 in signing a MASP transaction:
+                // the clients requests information to build SpendDescriptions.
+                // In particular, the ledger should answer with
+                // a proof generating key (PGK) and randomness (rcv and alpha)
+                // This APDU is called for each spend.
+                case INS_EXTRACT_SPEND: {
+                    CHECK_PIN_VALIDATED()
+                    handleExtractSpendDataMASPTransfer(tx, rx);
+                    break;
+                }
 #if defined(APP_TESTING)
                     case INS_TEST: {
                     handleTest(flags, tx, rx);
diff --git a/app/src/coin.h b/app/src/coin.h
index 881fcb5c..a6139640 100644
--- a/app/src/coin.h
+++ b/app/src/coin.h
@@ -100,10 +100,15 @@ typedef enum {
 #define INS_SIGN_WRAPPER                0x02
 
 #define INS_GET_SHIELDED_ADDRESS 0x10
+
 #define INS_INIT_MASP_TRANSFER 0xe0
+#define INS_EXTRACT_SPEND 0xe1
+
+
 #define INS_GET_IVK 0xf0
 #define INS_GET_OVK 0xf1
 #define INS_GET_NF 0xf2
+#define INS_GET_FVK 0xf3
 
 #define INS_GET_SIGNATURE 0x0A
 
diff --git a/app/src/crypto.c b/app/src/crypto.c
index 64e8e347..3f87a2f0 100644
--- a/app/src/crypto.c
+++ b/app/src/crypto.c
@@ -449,7 +449,7 @@ zxerr_t crypto_fillSaplingSeed(uint8_t *sk) {
 }
 
 
-// handleGetKeyFVK: return the full viewing key for a given path
+// handleGetKeyFVK: return the full viewing key (fvk = (ak, nk, ovk, dk)) for a given path
 zxerr_t crypto_fvk_sapling(uint8_t *buffer, uint16_t bufferLen, uint32_t p, uint16_t *replyLen) {
 
     zemu_log_stack("crypto_fvk_sapling");
@@ -648,5 +648,68 @@ zxerr_t crypto_hash_messagebuffer(uint8_t *buffer, uint16_t bufferLen, const uin
         return zxerr_unknown;
     }
     cx_hash_sha256(txdata, txdataLen, buffer, CX_SHA256_SIZE);      // SHA256
+    return zxerr_ok;
+}
+
+typedef struct {
+    union {
+        // STEP 1
+        struct {
+            uint8_t zip32_seed[ZIP32_SEED_SIZE];
+            uint8_t sk[ED25519_SK_SIZE];
+        } step1;
+
+        struct {
+            uint8_t ask[ASK_SIZE];
+            uint8_t nsk[NSK_SIZE];
+        } step2;
+    };
+} tmp_spendinfo_s;
+
+// handleExtractSpendDataMASPTransfer
+zxerr_t crypto_extract_spend_proof_key_and_rnd(uint8_t *buffer, uint16_t bufferLen){
+    // First check that there a still items on the list of spends
+    // for which spend data has not yet been extracted
+    if(!spendlist_more_to_extract()){
+        return zxerr_unknown;
+    }
+
+    // Ensure that the INS_INIT_MASP_TRANSFER has been called
+    // and did not error.
+    if(get_state() != STATE_PROCESSED_INPUTS){
+        return zxerr_unknown;
+    }
+
+    uint8_t *out = (uint8_t *) buffer;
+    MEMZERO(out, bufferLen);
+
+    // Get the next item from the list of spends
+    const spend_item_t *next = spendlist_extract_next();
+    if (next == NULL){
+        return zxerr_unknown;
+    }
+
+    tmp_spendinfo_s tmp = {0};
+
+    zxerr_t error = crypto_fillSaplingSeed(tmp.step1.zip32_seed);
+    CHECK_APP_CANARY()
+    if(error != zxerr_ok){
+        MEMZERO(buffer, bufferLen);
+        return error;
+    }
+
+    // Get ak and nsk (the child proof key)
+    get_child_proof_key(tmp.step1.zip32_seed, next->path, out, out + AK_SIZE);
+    CHECK_APP_CANARY()
+
+    MEMZERO(&tmp, sizeof(tmp_spendinfo_s));
+
+    MEMCPY(out+AK_SIZE+NSK_SIZE, next->rcmvalue, RCM_SIZE);
+    MEMCPY(out+AK_SIZE+NSK_SIZE+RCM_SIZE, next->alpha,ALPHA_SIZE);
+
+    if(!spendlist_more_to_extract()){
+        set_state(STATE_PROCESSED_SPEND_EXTRACTIONS);
+    }
+
     return zxerr_ok;
 }
\ No newline at end of file
diff --git a/app/src/crypto.h b/app/src/crypto.h
index 7d103bd7..b36ae3e7 100644
--- a/app/src/crypto.h
+++ b/app/src/crypto.h
@@ -50,6 +50,7 @@ zxerr_t crypto_getSignature(uint8_t *output, uint16_t outputLen, uint8_t slot);
 zxerr_t crypto_fvk_sapling(uint8_t *buffer, uint16_t bufferLen, uint32_t p, uint16_t *replyLen);
 zxerr_t crypto_extracttx_sapling(uint8_t *buffer, uint16_t bufferLen, const uint8_t *txdata, const uint16_t txdatalen);
 zxerr_t crypto_hash_messagebuffer(uint8_t *buffer, uint16_t bufferLen, const uint8_t *txdata, uint16_t txdataLen);
+zxerr_t crypto_extract_spend_proof_key_and_rnd(uint8_t *buffer, uint16_t bufferLen);
 
 #ifdef __cplusplus
 }
diff --git a/app/src/nvdata.c b/app/src/nvdata.c
index 4e4de3c9..355d2bb5 100644
--- a/app/src/nvdata.c
+++ b/app/src/nvdata.c
@@ -163,6 +163,28 @@ uint8_t spendlist_len() {
     return transaction_header.spendlist_len;
 }
 
+spend_item_t *spendlist_retrieve_item(uint8_t i) {
+    if (transaction_header.spendlist_len < i) {
+        return NULL;
+    } else {
+        return (spend_item_t *) &N_spendlist.items[i];
+    }
+}
+
+spend_item_t *spendlist_extract_next() {
+    if (transaction_header.spendlist_len <= transaction_header.spenddata_extract_index) {
+        return NULL;
+    } else {
+        spend_item_t *result = (spend_item_t *) &N_spendlist.items[transaction_header.spenddata_extract_index];
+        transaction_header.spenddata_extract_index += 1;
+        return result;
+    }
+}
+
+uint8_t spendlist_more_to_extract() {
+    return transaction_header.spendlist_len > transaction_header.spenddata_extract_index;
+}
+
 zxerr_t outputlist_append_item(uint8_t *d, uint8_t *pkd, uint64_t v, uint8_t memotype, uint8_t *ovk, uint8_t *rcmv,
                                uint8_t *rseed) {
     if (transaction_header.outputlist_len >= OUTPUT_LIST_SIZE) {
diff --git a/app/src/nvdata.h b/app/src/nvdata.h
index efe6392a..7e7ff9e4 100644
--- a/app/src/nvdata.h
+++ b/app/src/nvdata.h
@@ -121,6 +121,9 @@ zxerr_t t_outlist_append_item(uint8_t *addr, uint64_t v);
 // spend list nvdata functions
 zxerr_t spendlist_append_item(uint32_t p, uint64_t v, uint8_t *div, uint8_t *pkd, uint8_t *rcm, uint8_t *alpha);
 uint8_t spendlist_len();
+spend_item_t *spendlist_retrieve_item(uint8_t i);
+spend_item_t *spendlist_extract_next();
+uint8_t spendlist_more_to_extract();
 
 // shielded output nvdata functions
 zxerr_t outputlist_append_item(uint8_t *d, uint8_t *pkd, uint64_t v, uint8_t memotype, uint8_t *ovk, uint8_t *rcmv,
diff --git a/app/src/sapling.c b/app/src/sapling.c
index f1eeedc2..d9fa68db 100644
--- a/app/src/sapling.c
+++ b/app/src/sapling.c
@@ -633,7 +633,7 @@ void derive_dummy_ask_and_nsk(uint8_t *key_in, uint8_t *ask_out, uint8_t *nsk_ou
     MEMZERO(buffer, sizeof(buffer));
 }
 
-void get_fvk(uint8_t *seed, uint32_t pos, full_viewing_key_t* out){
+void get_all_keys(uint8_t *seed, uint32_t pos, all_keys_t* out){
     uint32_t path[3] = {FIRSTVALUE, COIN_TYPE, pos};
 
     uint8_t master_spending_key[64] = {0};
@@ -669,9 +669,9 @@ void get_fvk(uint8_t *seed, uint32_t pos, full_viewing_key_t* out){
             little_endian_write_u32((c+(1<<31)), le_i, sizeof (le_i));
             uint8_t *start_esk = expandedSpendingKey.ask;
             masp_blake2b_expand_vec_three(chain, sizeof (chain),
-                                         tmp_const, sizeof (tmp_const),
+                                          tmp_const, sizeof (tmp_const),
                                           start_esk, sizeof (expandedSpendingKey),
-                                         le_i, sizeof (le_i), tmp);
+                                          le_i, sizeof (le_i), tmp);
         } else {
             full_viewing_key_t fvk;
             ask_to_ak(expandedSpendingKey.ask, fvk.ak);
@@ -688,9 +688,9 @@ void get_fvk(uint8_t *seed, uint32_t pos, full_viewing_key_t* out){
             little_endian_write_u32(c, le_i, sizeof (le_i));
             uint8_t *start_fvk = fvk.ak;
             masp_blake2b_expand_vec_three(chain, sizeof (chain),
-                                         tmp_const, sizeof (tmp_const),
+                                          tmp_const, sizeof (tmp_const),
                                           start_fvk, sizeof (fvk),
-                                         le_i, sizeof (le_i), tmp);
+                                          le_i, sizeof (le_i), tmp);
         }
         memcpy(key, tmp,  32);
         memcpy(chain, tmp + 32,  32);
@@ -764,12 +764,31 @@ void get_fvk(uint8_t *seed, uint32_t pos, full_viewing_key_t* out){
         memcpy(expandedSpendingKey.dk, buffer, DK_SIZE);
         MEMZERO(buffer, sizeof(buffer));
     }
+    memcpy(out->ask, ask_bytes, ASK_SIZE);
+    memcpy(out->nsk, nsk_bytes, NSK_SIZE);
     ask_to_ak(ask_bytes, out->ak);
     nsk_to_nk(nsk_bytes, out->nk);
     memcpy(out->ovk, expandedSpendingKey.ovk, OVK_SIZE);
     memcpy(out->dk, expandedSpendingKey.dk, DK_SIZE);
 }
 
+void get_fvk(uint8_t *seed, uint32_t pos, full_viewing_key_t* out){
+    all_keys_t *keys = NULL;
+    get_all_keys(seed,pos,keys);
+    memcpy(out->ak, keys->ak, AK_SIZE);
+    memcpy(out->nk, keys->nk, NK_SIZE);
+    memcpy(out->ovk, keys->ovk, OVK_SIZE);
+    memcpy(out->dk, keys->dk, DK_SIZE);
+}
+
+
+void get_child_proof_key(uint8_t *seed, uint32_t pos, uint8_t* ak_out, uint8_t* nsk_out){
+    all_keys_t *keys = NULL;
+    get_all_keys(seed,pos,keys);
+    memcpy(ak_out, keys->ak, AK_SIZE);
+    memcpy(nsk_out, keys->nsk, NSK_SIZE);
+}
+
 
 void get_expanded_spending_key_from_seed(uint8_t *seed, expanded_spending_key_t* out){
     uint8_t master_spending_key[64] = {0};
diff --git a/app/src/sapling.h b/app/src/sapling.h
index 57848715..286a3c74 100644
--- a/app/src/sapling.h
+++ b/app/src/sapling.h
@@ -7,7 +7,10 @@ extern "C" {
 #include <stdint.h>   // uint*_t
 #include "sapling_keys.h"
 
- void get_fvk(uint8_t *seed, uint32_t pos, full_viewing_key_t* out);
+    void get_fvk(uint8_t *seed, uint32_t pos, full_viewing_key_t* out);
+
+    // Gets keys required for computing the zk proofs for spend descriptions (ak and nsk)
+    void get_child_proof_key(uint8_t *seed, uint32_t pos, uint8_t* ak_out, uint8_t* nsk_out);
 
 // Exists but not needed out of sapling.c yet:
 // static void get_expanded_spending_key_from_seed(uint8_t *seed, expanded_spending_key_t* out);
diff --git a/app/src/sapling_keys.h b/app/src/sapling_keys.h
index 1850beec..b8d92a11 100644
--- a/app/src/sapling_keys.h
+++ b/app/src/sapling_keys.h
@@ -21,4 +21,13 @@ typedef struct {
     uint8_t nk[NK_SIZE];    // nullifier deriving key
     uint8_t ovk[OVK_SIZE];  // outgoing viewing key
     uint8_t dk[DK_SIZE];    // diversifier key
-} full_viewing_key_t;
\ No newline at end of file
+} full_viewing_key_t;
+
+typedef struct {
+    uint8_t ak[AK_SIZE];
+    uint8_t ask[ASK_SIZE];
+    uint8_t nk[NK_SIZE];    // nullifier deriving key
+    uint8_t nsk[NSK_SIZE];
+    uint8_t ovk[OVK_SIZE];  // outgoing viewing key
+    uint8_t dk[DK_SIZE];    // diversifier key
+} all_keys_t;
\ No newline at end of file