diff --git a/bank-client.lua b/bank-client.lua
index a819933..84cde2e 100644
--- a/bank-client.lua
+++ b/bank-client.lua
@@ -9,6 +9,7 @@ local client = {}
 client.state = {
     auth = nil,
     hostId = nil,
+    securityKey = nil,
     timeout = 3
 }
 
@@ -28,18 +29,28 @@ local function request(command, data)
     return requestRaw({command = command, data = data})
 end
 
-local function requestAuth(command, data)
+local function requestAuth(command, data, secure)
+    secure = secure or false
     if not client.loggedIn() then
        return {success = false, error = "Client not logged in"} 
     end
-    return requestRaw({command = command, auth = client.state.auth, data = data})
+    local authInfo = {
+        username = client.state.auth.username,
+        password = client.state.auth.password
+    }
+    if secure and not client.state.securityKey then
+        return {success = false, error = "Missing security key for secure request."}
+    end
+    autInfo.key = client.state.securityKey
+    return requestRaw({command = command, auth = authInfo, data = data})
 end
 
 -- Base functions
 
-function client.init(modemName, host)
+function client.init(modemName, host, securityKey)
     rednet.open(modemName)
     client.state.hostId = rednet.lookup("BANK", host)
+    client.state.securityKey = securityKey or nil
     return client.state.hostId ~= nil
 end
 
@@ -103,4 +114,12 @@ function client.renameAccount(accountId, newName)
     return response.success, response.error
 end
 
+function client.recordTransaction(accountId, amount, description)
+    local response = requestAuth("RECORD_TRANSACTION", {accountId = accountId, amount = amount, description = description}, true)
+    if not response.success then
+        return nil, response.error
+    end
+    return response.data
+end
+
 return client
diff --git a/bank.lua b/bank.lua
index 89c62e4..d9d29d6 100644
--- a/bank.lua
+++ b/bank.lua
@@ -55,6 +55,10 @@ local function validateUsername(name)
     )
 end
 
+local function validateTransactionDescription(desc)
+    return string.find(desc, "^%w+[ !%.%w]*$") ~= nil and #desc <= 64
+end
+
 local function userDir(name)
     return fs.combine(USERS_DIR, name)
 end
@@ -102,12 +106,28 @@ local function saveAccounts(name, accounts)
     writeJSON(userAccountsFile(name), accounts)
 end
 
+local function findAccountById(accounts, id)
+    for i, account in pairs(accounts) do
+        if account.id == id then
+            return account
+        end
+    end
+    return nil
+end
+
+local function findAccountByName(accounts, name)
+    for i, account in pairs(accounts) do
+        if account.name == name then
+            return account
+        end
+    end
+    return nil
+end
+
 local function createAccount(username, accountName)
     local accounts = getAccounts(username)
-    for i, account in pairs(accounts) do
-        if account.name == accountName then
-            return false, "Duplicate account name"
-        end
+    if findAccountByName(accounts, accountName) then
+        return false, "Duplicate account name"
     end
     local newAccount = {
         id = randomAccountId(),
@@ -140,15 +160,11 @@ end
 
 local function renameAccount(username, accountId, newName)
     local accounts = getAccounts(username)
-    local targetAccount = nil
-    for i, account in pairs(accounts) do
-        if account.id == accountId then
-            targetAccount = account
-        elseif account.name == newName then
-            return false, "Duplicate account name"
-        end
-    end
+    local targetAccount = findAccountById(accounts, accountId)
     if not targetAccount then return false, "Account not found" end
+    if findAccountByName(accounts, newName) ~= nil then
+        return false, "Duplicate account name"
+    end
     targetAccount.name = newName
     saveAccounts(accounts)
     log("Renamed user " .. username .. " account " .. accountId .. " to " .. newName)
@@ -188,6 +204,32 @@ local function renameUser(oldName, newName)
     return true
 end
 
+local function recordTransaction(username, accountId, amount, description)
+    if not validateTransactionDescription(description) then return false, "Invalid transaction description" end
+    if not userExists(username) then return false, "User doesn't exist" end
+    local accounts = getAccounts(username)
+    local account = findAccountById(accounts, accountId)
+    if account == nil then return false, "Account doesn't exist" end
+    if account.balance + amount < 0 then return false, "Insufficient funds" end
+    -- Everything is OK, record the transaction.
+    local tx = {
+        amount = amount,
+        description = description,
+        timestamp = os.epoch("utc")
+    }
+    local f = io.open(accountTransactionsFile(username, accountId), "a")
+    local txStr = tostring(tx.amount)..";"..tostring(tx.timestamp)..";"..tx.description
+    f:write(txStr .. string.rep(" ", 99 - #txStr) .. "\n")
+    f:close()
+    if fs.getSize(accountTransactionsFile(username, accountId)) % 100 ~= 0 then
+        log("WARNING! Transaction file for account " .. accountId .. " is not consistent!")
+    end
+    account.balance = account.balance + amount
+    saveAccounts(username, accounts)
+    os.queueEvent("bank_account_balance", username, accountId, account.balance)
+    return true, tx
+end
+
 local function initSecurityKey()
     -- Initialize security key
     local SECURITY_KEY_FILE = "key.txt"
@@ -290,6 +332,17 @@ local function handleRenameUserAccount(msg)
     return {success = success, error = errorMsg}
 end
 
+local function handleRecordTransactionToAccount(msg)
+    if not msg.data or not msg.data.amount or not msg.data.description or not msg.data.accountId then
+        return {success = false, error = "Invalid request. Requires data.amount and data.description and data.accountId."}
+    end
+    local success, errorMsgOrTx = recordTransaction(msg.auth.username, msg.data.accountId, msg.data.amount, msg.data.description)
+    if not success then
+        return {success = false, error = errorMsgOrTx}
+    end
+    return {success = true, data = errorMsgOrTx}
+end
+
 -- A registry of all possible BANK requests, and their handler functions.
 local BANK_REQUESTS = {
     ["STATUS"] = handleGetStatus,
@@ -299,7 +352,8 @@ local BANK_REQUESTS = {
     ["GET_ACCOUNTS"] = authProtect(handleGetUserAccounts),
     ["CREATE_ACCOUNT"] = authProtect(handleCreateUserAccount),
     ["DELETE_ACCOUNT"] = authProtect(handleDeleteUserAccount),
-    ["RENAME_ACCOUNT"] = authProtect(handleRenameUserAccount)
+    ["RENAME_ACCOUNT"] = authProtect(handleRenameUserAccount),
+    ["RECORD_TRANSACTION"] = authProtect(handleRecordTransactionToAccount, true)
 }
 
 local function handleBankMessage(remoteId, msg)